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The Origin of vertebrates 



Walter Holbrook Gaske 



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THE 

ORIGIN OF VERTEBRATES 



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THE 

ORIGIN OF VERTEBRATES 



BY 

WALTER HOLBROOK GASKELL 

M.A., M.D. (CANTAB.), LL.D. (EDIN. AND McGILL UNIV.) ; F.R.S. ; FELLOW OF TRINITY 
HALL AND UNIVERSITY LECTURER IN PHYSIOLOGY, CAMBRIDGE ; HONORARY FELLOW 
OF THE ROYAL MEDICAL AND CHIRURGICAL SOCIETY; CORRESPONDING MEMBER 
OF THE IMPERIAL MILITARY ACADEMY OF MEDICINE, ST. PETERSBURG, ETC. 



LONGMANS, GREEN, AND CO. 

39 PATERNOSTER ROW, LONDON 

NEW YORK, BOMBAY, AND CALCUTTA 

1908 

All rights reserved 

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CONTENTS 



PAGE 

Intboduction 1 

CHAPTER I 

The Evidence op the Central Nervous System 

Theories of the origin of vertebrates — Importance of the central nervous system 
— Evolution of tissues — Evidence of Palteontology — Reasons for choosing 
Ammocoetes rather than Amphioxus for the investigation of this problem — 
Importance of larval forms — Comparison of the vertebrate and arthropod 
central nervous systems — Antagonism between cephalization and alimenta- 
tion — Life-history of lamprey, not a degenerate animal — Brain of Ammo- 
coetes compared with brain of arthropod — Summary 8 

CHAPTER II 
The Evidence op the Organs op Vision 

Different kinds of eye — Simple and compound retinas — Upright and inverted 
retinas — Median eyes — Median or pineal eyes of Ammoccetes and their 
optic ganglia — Comparison with other median eyes — Lateral eyes of verte- 
brates compared with lateral eyes of crustaceans— Peculiarities of the 
lateral eye of the lamprey— Meaning of the optic diverticula— Evolution 
of vertebrate eyes — Summary 68 

CHAPTER III 

The Evidence op the Skeleton 

The bony and cartilaginous skeleton considered, not the notochord — Nature of 
the earliest cartilaginous skeleton — The mesosomatic skeleton of Ammo- 
coetes; its topographical arrangement, its structure, its origin in muco- 
cartilage— The prosomatic skeleton of Ammocoetes ; the trabecule and 
parachordals, their structure, their origin in white fibrous tissue — Tho 
mesosomatic skeleton of Limulus compared with that of Ammocoetes ; 
similarity of position, of structure, of origin in muco-cartilage — The 
prosomatic skeleton of Limulus; the entosternite, or plastron, compared 
with the trabecules of Ammoccetes ; similarity of position, of structure, of 
origin in fibrous tissue— Summary 119 



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vi CONTENTS 

CHAPTER IV 
The Evidence op the Respiratory Apparatus 

l'AGE 

Branchiae considered as internal branchial appendages— Innervation of branchial 
segments— Cranial region older than spinal— Three-root system of cranial 
nerves: dorsal, lateral, ventral— Explanation of van Wijhe's segments- 
Lateral mixed root is appendage-nerve of invertebrate — The branchial 
chamber of Ammocoetes — The branchial unit, not a pouch but an appendage 
— The origin of the branchial musculature— The branchial circulation— The 
branchial heart of the vertebrate— Not homologous with the systemic heart 
of the arthropod— Its formation from two longitudinal venous sinuses — 
Summary ........••••• *^° 



CHAPTER V 
The Evidence op the Thyroid Gland 

The value of the appendage-unit in non-branchial segments — The double nature 
of the hyoid segment— Its branchial part— Its thyroid part— The double 
nature of the opercular appendage— Its branchial part— Its genital part- 
Unique character of the thyroid gland of Ammocoetes— Its structure- 
Its openings — The nature of the thyroid segment— The uterus of the 
scorpion— Its glands — Comparison with the thyroid gland of Ammocoetes— 
Cephalic generative glands of Limulus— Interpretation of glandular tissue 
filling up the brain-case of Ammocoetes— Function of thyroid gland — 
Relation of thyroid gland to sexual functions— Summary .... 185 



CHAPTER VI 

The Evidence of the Olfactory Apparatus 

Fishes divided into Amphirhin® and Monorhinse— Nasal tube of the lamprey 
—Its termination at the infundibulum— The olfactory organs of the scorpion 
group— The camerostome— Its formation as a tube— Its derivation from a 
pair of antenna*— Its termination at the true mouth— Comparison with the 
olfactory tube of Ammocoetes— Origin of the nasal tube of Ammocoetes from 
the tube of the hypophysis— Direct comparison of the hypophysial tube with 
the olfactory tube of the scorpion group — Summary 21 8 



CHAPTER VII 
The Prosomatic Segments of Limulus and its Allies 

Comparison of the trigeminal with the prosomatic region— The prosomatic 
appendages of the Gigantostraca— Their number and nature— Endognaths 
and ectognath— The metastoma— The coxal glands- Prosomatic region of 
Eurypterus compared with that of Ammocoetes— Prosomatic segmentation 
shown by marks on carapace— Evidence of coelomic cavities in Limulus— 
Summary 



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CONTENTS vii 

CHAPTER VIII 
Thb Segments belonging to the Trigeminal Nerve-Group 

l'AGE 

The prosomatic segments of the vertebrate — Number of segments belonging to 
the trigeminal nerve-group — History of cranial segments — Eye-muscles and 
their nerves — Comparison with the dorso-ventral somatic muscles of the 
scorpion — Explanation of the oculomotor nerve and its group of muscles — 
Explanation of the trochlear nerve and its dorsal crossing — Explanation 
of the abducens nerve — Number of segments supplied by the trigeminal 
nerves — Evidence of their motor nuclei — Evidence of their sensory ganglia 
— Summary 267 

CHAPTER IX 
The Prosomatic Segments of Ammoccetes 

The prosomatic region in Ammoccetes — The suctorial apparatus of the adult 
Petromyzon — Its origin in Ammoccetes — Its derivation from appendages — 
The segment of the lower lip or the metastomal segment— The tentacular 
segments — The tubular muscles — Their segmental arrangement — Their 
peculiar innervation — Their correspondence with the system of veno-peri- 
cardial muscles in Limulus — The old mouth or palseostoma — The pituitary 
gland — Its comparison with the coxal gland of Limulus— Summary . . 286 

CHAPTER X 
The Relationship op Ammoccetes to the most Ancient Fishes 

— THE OSTRACODERMATA 

The nose of the Osteostraci — Comparison of head-shield of Ammoccetes and of 
Cephalaspis — Ammoccetes only living representative of these ancient fishes 
— Formation of cranium — Closure of old mouth — Rohon's primordial 
cranium — Primordial cranium of Phrynus and Galeodes— Summary . . 326 

CHAPTER XI 

The Evidence of the Auditory Apparatus and the Organs of 

the Lateral Line 

Lateral line organs — Function of this group of organs— Poriferous sense-organs 
on* the appendages in Limulus— Branchial sense-organs — Prosomatic sense- 
organs — Flabellum— Its structure and position— Sense-organs of mandibles 
— Auditory organs of insects and arachnids — Poriferous chordotonal organs — 
Balancers of Diptera — Resemblance to organs of flabellum— Racquet-organs 
of Galeodes — Pectens of scorpions— Large size- of nerve to all these special 
sense-organs — Origin of parachordals and auditory capsule — Reason why 
VHth nerve passes in and out of capsule — Evidence of Ammoccetes — 
Intrusion of glandular mass round brain into auditory capsule— Intrusion 
of generative and hepatic mass round brain into base of flabellum — 
Summary 355 



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viii CONTENTS 

CHAPTER XII 
The Region op the Spinal Cord 

PAG* 

Difference between cranial and spinal regions — Absence of lateral root — Meristic 
variation — Segmentation of coelom — Segmental excretory organs — Develop- 
ment of nephric organs ; pronephric, mesonephric, metanepbric— Excretory 
organs of Ampbioxus— Solenocytes — Excretory organs of Brancbipus and 
Peripatus, appendicular and somatic — Comparison of ccelom of Peripatus 
and of vertebrate — Pronepbric organs compared to coxal glands — Origin of 
vertebrate body-cavity (metaccele) — Segmental duct — Summary of formation 
of excretory organs — Origin of somatic trunk-musculature — Atrial cavity 
of Ampbioxus — Pleural folds — Ventral growth of pleural folds and somatic 
musculature -Pleural folds of Cephalaspidse and of Trilobita — Meaning of 
the ductless glands — Alteration in structure of excretory organs whicb bave 
lost their duct in vertebrates and in invertebrates — Formation of lymphatic 
glands — Segmental coxal glands of arthropods and of vertebrates — Origin of 
adrenals, pituitary body, thymus, tonsils, thyroid, and other ductless glands 
— Summary 385 



CHAPTER XIII 
The Notochord and Alimentary Canal 

Relationship between notochord and gut — Position of unsegmented tube of 
notochord— Origin of notochord from a median groove — Its function as an 
accessory digestive tube — Formation of notochordal tissue in invertebrates 
from closed portions of the digestive tube — Digestive power of the skin of 
Ammocoetes—Formation of new gut in Ammocoetes at transformation — 
Innervation of the vertebrate gut— The throe outflows of efferent nerves 
belonging to the organic system — The original close contiguity of the 
respiratory chamber to the cloaca— The elongation of the gut— Conclusion 433 



CHAPTER XIV 

The Principles op Embryology 

The law of recapitulation— Vindication of this law by the theory advanced in 
this book— The germ-layer theory — Its present position — A physiological 
not a morphological conception- New fundamental law required — Com- 
position of adult body- Neuroepithelial syncytium and free-living cells- 
Meaning of the blastula- Derivation of the Metazoa from the Protozoa — 
Importance of the central nervous system for Ontogeny as well as for 
Phylogeny — Derivation of free-living cells from germ-cells -Meaning of 
ccelom— Formation of neural canal — Gastrula of Ampbioxus and of Lucifer 
—Summary 455 



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CONTENTS ix 

CHAPTER XV 
Final Remarks 

PAGE 

Problems requiring investigation — 

Giant nerve-cells and giant nerve-fibres; their comparison in fishes and 
arthropods ; blood- and lymph-corpuscles ; nature of the skin ; origin of 
system of unstriped muscles ; origin of the sympathetic nervous system ; 
biological test of relationship. 

Criticisms of Balanoglossus theory — Theory of parallel development— Importance 

of the theory advocated in this book for all problems of Evolution . . 488 



Bibliography and Index of Authors 501 

General Index 517 



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"GO ON AND PROSPER; THERE IS NOTHING SO 
USEFUL IN SCIENCE AS ONE OF THOSE EARTH- 
QUAKE HYPOTHESES, WHICH OBLIGE ONE TO FACE 
THE POSSIBILITY THAT THE SO LID EST- LOO KING 
STRUCTURES MAY COLLAPSE." 



Letter from Prof. Huxley to 
the Author. June 2, 1889. 



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THE 

ORIGIN OF VERTEBRATES 

INTRODUCTION 

In former days it was possible for a man like Johannes Muller 
to be a leader both in physiology and in comparative anatomy. 
Nowadays all scientific knowledge has increased so largely that 
specialization is inevitable, and every investigator is confined more 
and more not only to one department of science, but as a rule to 
one small portion of that department. In the case of such cognate 
sciences as physiology and comparative anatomy this limiting of the 
scope of view is especially deleterious, for zoology without physiology 
is dead, and physiology in many of its departments without com- 
parative anatomy can advance but little. Then, again, the too 
exclusive study of one subject always tends to force the mind into 
a special groove— into a line of thought so deeply tinged with the 
prevalent teaching of the subject, that any suggestions which arise 
contrary to such teaching are apt to be dismissed at once as heretical 
and not worthy of further thought ; whereas the same suggestion 
arising in the mind of one outside this particular line of thought 
may give rise to new and valuable scientific discoveries. 

Nothing but good can, in my opinion, result from the incursion 
of the non-specialist into the realm of the specialist, provided that 
the former is in earnest. Over and over again the chemist has 
given valuable help to the physicist, and the physicist to the 
chemist, so closely allied are the two subjects; so also is it with 
physiology and anatomy, the two subjects are so interdependent 
that a worker in the one may give valuable aid towards the solution 
of some large problem which is the special territory of the other. 

It has been a matter of surprise to many how it came about that 

D 



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2 THE ORIGIN OF VERTEBRATES 

I, a worker in the physiological laboratory at Cambridge ever since 
Foster introduced experimental physiology into English-speaking 
nations, should have devoted so much time to the promulgation of 
a theory of the origin of vertebrates — a subject remote from phy- 
siology, and one of the larger questions appertaining to comparative 
anatomy. By what process of thought was I led to take up the 
consideration of a subject apparently so remote from all my previous 
work, and so foreign to the atmosphere of a physiological laboratory ? 

It may perhaps be instructive to my readers to see how one 
investigation leads to another, until at last, nolens volem, the worker 
finds himself in front of a possible solution to a problem far removed 
from his origiual investigation, which by the very magnitude and 
importance of it forces him to devote his whole energy and time to 
seeing whether his theory is good. 

In the years 1880-1884 I was engaged in the investigation of 
the action of the heart, and the nature of the nerves which regulate 
that action. In the course of that investigation I was struck by the 
ease with which it was possible to distinguish between the fibres of 
the vagus and accelerator nerves on their way to the heart, owing to 
the medullation of the former and the non-medullation of the latter. 
This led me to an investigation of the accelerator fibres, to find out 
how far they are non-medullated, and so to the discovery that the 
rami communicantes connecting together the central nervous system 
and the sympathetic are in reality single, not double, as had 
hitherto been thought; for the grey ramus commnnicans is in 
reality a peripheral nerve which supplies the blood-vessels of the 
spinal cord and its membranes, and is of the same nature as the 
grey accelerators to the heart. 

This led to the conclusion that there is no give and take 
between two independent nervous systems, the cerebro-spinal and 
the sympathetic, as had been taught formerly, but only one nervous 
system, the cerebro-spinal, which sends special medullated nerve- 
fibres, characterized by their smallness, to the cells of the sympathetic 
system, from which fibres pass to the periphery, usually non- 
medullated. These fine medullated nerves form the system of 
white rami communicantes, and have since been called by Langley 
the preganglionic nerves. Further investigation showed that such 
white rami are not universally distributed, but are confined to the 
thoracico-lumbar region, where their distribution is easily seen in 



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INTRODUCTION 3 

the ventral roots, for the cells of the sympathetic system are entirely 
efferent in nature, not afferent ; therefore, the fibres entering into them 
from the central nervous system leave the spinal cord by ventral, not 
dorsal roots. 

Following out this clue, I then found that in addition to this 
thoracico-lumbar outflow of efferent ganglionated visceral nerves, 
there are similar outflows in the cranial and sacral regions, belong- 
ing in the former case especially to the vagus system of nerves, and 
in the latter to the system of nerves which pass from the sacral 
region of the cord to the ganglion-cells of the hypogastric plexus, 
and from them supply the bladder, rectum, etc. To this system of 
nerves, formerly called the nervi erigentes, I gave the name pelvic 
splanchnics, in order to show their uniformity with the abdominal 
splanchnics. These investigations led to the conclusion that the 
organic system of nerves, characterized by the possession of efferent 
nerve- cells situated peripherally, arises from the central nervous 
system by thrde distinct outflows— cranial, thoracico-lumbar, and 
sacral, respectively. To this system Langley has lately given the 
name * autonomic.' These three outflows are separated by two gaps 
just where the plexuses for the anterior and posterior extremities 
come in. 

This peculiar arrangement of the white rami communicant cs set 
me thinking, for the gaps corresponded to an increase of somatic 
musculature to form the muscles of the fore and hind limbs, so that 
if, as seemed probable, the white rami communicantes arise segmentally 
from the spinal cord, then a marked distinction must exist in 
structure between the spinal cord in the thoracic region, where the 
visceral efferent nerves are large in amount and the body muscu- 
lature scanty, and in the cervical or lumbar swellings, where the 
somatic musculature abounds, and the white rami communicantes 
scarcely exist. 

I therefore directed my attention in the next place to the 
structure of the central nervous system in the endeavour to asso- 
ciate the topographical arrangement of cell-groups in this system 
with the outflow of the different kinds of nerve-fibres to the 
peripheral organs. 

This investigation forcibly impressed upon my mind the 
uniformity in the arrangement of the central nervous system as far 
as the centres of origin of all the segmental nerves are concerned, 



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4 THE O RIG IS OF VERTEBRATES 

S//M cranial and spinal, and alv> the original segmental character of 
thi* part of tith nervou* *y?:em. 

I co*;ld BK^t, tlierefore, help being struck by the fon* of the 
comparison b**we*?n the central nervoos systems of YertebraU and 
Apl^ndiculata as put forward again and again by the past gene- 
ration of comparative anatomists, and wondered why it had been 
diwedited. There in the infundibulum was the old cesophagus, 
there in the cranial segmental nerves the infraoesophageal ganglia, 
there in the cerebral hemispheres and optic and olfactory nerves the 
ftupnu£*ophageal ganglia, there in the spinal cord the ventral chain 
of ganglia. But if the infundibulum was the old cesophagus, what 
then ? The old oesophagus was continuous with and led into the 
ceplialic stomach. What about the infundibulum ? It was continuous 
with and led into the ventricles of the brain, and the whole thing 
\wdu\<i clear. The ventricles of the brain were the old cephalic 
atomach, and the canal of the spinal cord the long straight intestine 
which led originally to the anus, and still in the vertebrate embryo 
oj>cn* out into the anus. Not having been educated in a morpho- 
logical laboraU/ry and taught that the one organ which is homologous 
throughout the animal kingdom is the gut, and that therefore the 
gut of the invertebrate ancestor must continue on as the gut of 
the vcrtel/rate, the conception that the central nervous system has 
j/rown round and enclosed the original ancestral gut, and that the 
v<;rtehrate has formed a new gut did not seem to me so impossible 
a* to prevent my taking it as a working hypothesis, and seeing to 
what it would lead. 

This theory that the so-called central nervous system of the 
vertebrate is in reality composed of two separate parts, of which 
the one, the Begmentcd part, corresponds to the central nervous 
«ystem of the highest invertebrates, while the other, the unseg- 
inented tube, was originally the alimentary canal of that same 
invertebrate, came into my mind in the year 1887. The following 
your, on June 23, 1888, I read a paper on the subject before the 
Anatomical Society at Cambridge, which was published in the Journal 
of Anatomy and Phytdology, vol. 23, and more fully in the Journal of 
Vhyriology, vol. 10. Since that time I have l)een engaged in testing 
the theory in every possible way, and have published the results of 
my investigations in a series of papers in different journals, a list of 
which 1 appuud at the end of this introductory chapter. 



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INTRODUCTION 5 

It is now twenty years since the theory first came into my mind, 
and the work of those twenty years has convinced me more and more 
of its truth, and yet during the whole time it has been ignored by 
the morphological world as a whole rather than criticized. Whatever 
may have been the causes for such absence of criticism, it is clear 
that the serial character of its publication is a hindrance to criticism 
of the theory as a whole, and I hope, therefore, that the publication 
of the whole of the twenty years' work in book-form will induce 
those who differ from my conclusions to come forward and show me 
where I am wrong, and why my theory is untenable. Any one 
who has been thinking over any one problem for so long a time 
becomes obsessed with the infallibility of his own views, and is not 
capable of criticizing his own work as thoroughly as others would 
do. I have been told that it is impossible for one man to consider 
so vast a subject with that thoroughness which is necessary, before 
any theory can be accepted as the true solution of the problem. I 
acknowledge the vastness of the task, and feel keenly enough my 
own shortcomings. For all that, I do feel that it can only be of 
advantage to scientific progress and a help to the solution of this 
great problem, to bring together in one book all the facts which I 
have been able to collect, which appeal to me as having an important 
bearing on this solution. 

In this work I have been helped throughout by Miss E. Alcock. 
It is not too much to say that without the assistance she has given 
me, many an important link in the chain of evidence would have 
been missing. With extraordinary patience she has followed, section 
by section, the smallest nerves to their destination, and has largely 
helped to free the transformation process in the lamprey from the 
mystery which has hitherto enveloped it. She has drawn for me 
very many of the illustrations scattered through the pages in this 
book, and I feel that her aid has been so valuable and so continuous, 
lasting as it does over the whole period of the work, that her name 
ought fittingly to be associated with mine, if perchance the theory of 
the Origin of Vertebrates, advocated in the pages of this book, gains 
acceptance. 

I am also indebted to Mr. J. Stanley Gardiner and to Dr. A. 
Sheridan Lea for valuable assistance in preparing this book for the 
press. I desire to express my grateful thanks to the former for 
valuable criticism of the scientific evidence which I have brought 



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6 THE ORIGIN OF VERTEBRATES 

forward in this book, and to the latter for his great kindness in 
undertaking the lal>orious task of correcting the proofs. 



LIST OP PREVIOUS PUBLICATIONS BY THE AUTHOR, CON- 
CERNING THE ORIGIN OP VERTEBRATES. 

18K8. •• Spinal and Cranial Nerves." Proceedings of the Anatomical Society, 
June, 1888. Journal of Anatomy and Physiology, vol. xxiii. 

1889. ** On the Relation between the Structure. Function, Distribution, and 
Origin of the Cranial Nerves ; together with a Theory of the Origin 
of the Nervous System of Vertebrata." Journal of Physiology, vol. x., 
p. 153. 

1889. 4, On the Origin of the Central Nervous System of Vertebrates." 

Brain, vol. xii., p. 1. 

1890. "On the Origin of Vertebrates from a Crustacean-like Ancestor." 

Quarterly Journal of Microscopical Science, vol. xxxi.. p. 379. 

1895. "The Origin of Vertebrates." Proceedings of the Cambridge Philo- 
sophical Society, vol. ix., p. 19. 

189*5. Presidential Address to Section I. at the meeting of the British 
Association for the Advancement of Science in Liverpool. Report 
of the British Association, 1896. p. 942. 

1899. " On the Meaning of the Cranial Nerves." Presidential Address to the 
Neurological Society for the year 1899. Brain, vol. xxii.. p. 329. 

A series of papers on "The Origin of Vertebrates, deduced from the 
study of Ammoccetes," in the Journal of Anatomy and Physiology, as 
follows : — 

1898. Part I. " The Origin of the Brain." vol. xxxii., p. 513. 

II. "The Origin of the Vertebrate Cranio-facial Skeleton." 
vol. xxxii.. p. 553. 
III. " The Origin of the Branchial Segmentation," vol. xxxiii.. 
p. 1>4. 

1899. IV. " The Thyroid, or Opercular Segment : the Meaning of the 

Facial Nerve." vol. xxxiii.. p. tf3S. 
19»*>. V. "The Origin of the Pro-otic Segmentation: the Meaning 

of the Trigeminal and Eye-muscle Nerves." vol. xxxi v.. 

p. 4^5. 
19»*>. VI. "The Old Mouth and the Olfactory Organ : the Meaning 

of the First Nerve." vol xxxiv.. p. 514. 
19<*>. VII. " The Evidence of Prosomatic Appendages in Ammocoetes, 

as given by the Course and Distribution of the Trigeminal 

Nerve." voL xxxiv.. p. 537. 
19U>. .. VIII. "The Palieontological Evidence: Ammoecetes a Cepha- 

laspid." vol. xxxiv., p. 5*>2. 
19*»1. IX. "The Origin of the Optic Apparatus: th* Meaning of the 

Optic XervwO* vol. xxxv.. p. 224. 



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INTRODUCTION J 

1902. Part X. " The Origin of the Auditory Organ : the Meaning of the 

Vlllth Cranial Nerve," vol. xxxvi., p. 164. 

1903. „ XI. " The Origin of the Vertebrate Body-cavity and Excretory 

Organs : the Meaning of the Somites of the Trunk and 
of the Ductless Glands," vol. xxxvii., p. 168. 

1905. „ XII. " The Principles of Embryology," vol. xxxix., p. 371. 

1906. „ XIII. " The Origin of the Notochord and Alimentary Canal," 

vol. xl., p. 305. 



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CHAPTER I 

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 

Theories of the origrin of vertebrates. — Importance of the central nervous 
system. — Evolution of tissues. — Evidence of Paheontology . — Reasons for 
choosing Ammocoetes rather than Amphioxus. — Importance of larval forms. 
— Comparison of the vertebrate and arthropod central nervous systems. — 
Antagonism between cephalization and alimentation. — Life-history of 
lamprey : not a degenerate animal.— Brain of Ammocoetes compared with 
brain of arthropod. — Summary. 

At the present time it is no longer a debatable question whether or 
no Evolution has taken place. Since the time of Darwin the accu- 
mulation of facts in its support has been so overwhelming that all 
zoologists look upon this question as settled, and desire now to find 
out the manner in which such evolution has taken placa Here two 
problems offer themselves for investigation, which can be and are 
treated separately — the one dealing with the question of those laws 
of heredity and variation which have brought about in the past and 
are still causing in the present the evolution of living beings, i.e. the 
causes of evolution; the other concerned with the relationship of 
animals, or groups of animals, rather than with the causes which 
have brought about such relationship, i.e. the sequence of evolution. 

It is the latter problem with which this book deals, and, indeed, 
not with the whole question at all, but only with that part of it 
which concerns the origin of vertebrates. 

This problem of the sequence of evolution is of a twofold character : 
first, the finding out of the steps by which the higher forms in 
any one group of animals have been evolved from the lower ; and 
secondly, the evolution of the group itself from a lower group. 

In any classification of the animal kingdom, it is clear that large 
groups of animals exist which have so many common characteristics 
as to necessitate their being placed in one larger group or kingdom ; 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 9 

thus zoologists are able to speak definitely of the Vertebrata, Arthro- 
poda, Annelida, Echinodermata, Porifera, Ccelenterata, Mollusca, 
etc. In each of these groups affinities can be traced between the 
members, so that it is possible to speak of the progress from lower 
to higher members of the group, and it is conceivable, given time to 
work out the details, that the natural relationships between the 
members of the whole group will ultimately be discovered. 

Thus no one can doubt that a sequence of the kind has taken 
place in the Vertebrata as we trace the progress from the lowest fishes 
to man, and already the discoveries of palaeontology and anatomy 
give us a distinct clue to the sequence from fish to amphibian, from 
amphibian to reptile, from reptile to mammal on the one hand, and 
to bird on the other. That the different members of the vertebrate 
group are related to each other in orderly sequence is no longer a 
matter of doubt ; the connected problems are matters of detail, the 
solution of which is certain sooner or later. The same may be said 
of the members of any of the other great natural groups, such as 
the Arthropoda, the Annelida, the Echinodermata, etc. 

It is different, however, when an attempt is made to connect 
two of the main divisions themselves. It is true enough that there 
is every reason to believe that the arthropod group has been evolved 
from the segmented annelid, and so the whole of the segmented 
invertebrates may be looked on as forming one big division, the 
Appendiculata, all the members of which will some day be arranged 
in orderly sequence, but the same feeling of certainty does not exist 
in other cases. 

In the very case of the origin of the Appendiculata we are con- 
fronted with one of the large problems of evolution — the origin of 
segmented from non-segmented animals — the solution of which is not 
yet known. 



Theories of the Origin of Vertebrates. 

The other large problem, perhaps the most important of all, is the 
question of the relationship of the great kingdom of the Vertebrata : 
from what invertebrate group did the vertebrate arise ? 

The great difficulty which presents itself in attempting a solution 
of this question is not so much, as used to be thought, the difficulty 
of deriving a group of animals possessing an internal bony and 



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IO 



THE ORIGIN OF VERTEBRATES 



cartilaginous skeleton from a group possessing an external skeleton 
of a calcareous or chitinous nature, but rather the difficulty caused by 
the fundamental difference of arrangement of the important internal 
organs, especially the relative positions of the central nervous system 
and the digestive tube. 

Now, if we take a broad and comprehensive view of the inver- 
tebrate kingdom, without arguing out each separate case, we find that 





Fig. 1. — Arrangement op Organs in the Vertebrate (A) and Arthropod (B)- 
Al, gut; H 9 heart; C.N.S., central nervous system ; V, ventral side ; D, dorsal side. 

it bears strongly the stamp of a general plan of evolution derived 
from a coelenterate animal, whose central nervous system formed a 
ring surrounding the mouth. Then when the radial symmetry was 
given up, and an elongated, bilateral, segmented form evolved, the 
central nervous system also became elongated and segmented, but, 
owing to its derivation from an oral ring, it still surrounded the 
mouth-tube, or oesophagus, and thus in its highest forms is divided into 
supra-cesophageal and infra-oesophageal nervous masses. These latter 



"V 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM II 

nervous masses are of necessity ventral to the digestive tube, because 
the mouth of the coelenterate is on the ventral side. The striking 
characteristic, then, of the invertebrate kingdom is the situation of a 
large portion of the central nervous system ventrally to the alimentary 
canal and the piercing of the nervous system by a tube — the (eso- 
phagus — leading from the mouth to the alimentary canal. The 
equally striking characteristic of the vertebrate is the dorsal position 
of the central nervous system and the ventral position of the ali- 
mentary canal combined with the absence of any piercing of the 
central nervous system by the oesophagus. 

So fundamentally different is the arrangement of the important 
organs in the two groups that it might well give rise to a feeling of 
despair of ever hoping to solve the problem of the Origin of Verte- 
brates; and, to my mind, this is the prevalent feeling among 
morphologists at the present time. Two attempts at solution have 
been made. The one is associated with the name of Geoffrey St. 
Hilaire, and is based on the supposition that the vertebrate has 
arisen from the invertebrate by turning over on its back, swimming 
in this position, and so gradually converting an originally dorsal 
surface into a ventral one, and vice versa ; at the same time, a new 
mouth is supposed to have been formed on the new ventral side, 
which opened directly into the alimentary canal, while the old 
mouth, which had now become dorsal, was obliterated. 

The other attempt at solution is of much more recent date, and is 
especially associated with the name of Bateson. It supposes that 
bilaterally symmetrical, elongated, segmented animals were formed 
from the very first in two distinct ways. In the one case the diges- 
tive tube pierced the central nervous system, and was situated dorsally 
to its main mass. In the other case the segmented central nervous 
system was situated from the first dorsally to the alimentary canal, 
and was not pierced by it. In the first case the highest result of 
evolution led to the Arthropoda ; in the second case to the Vertebrata. 

Neither of these views is based on evidence so strong as to cause 
universal acceptance. The great difficulty in the way of accepting 
the second alternative is the complete absence of any evidence, either 
among animals living on the earth at the present day or among those 
known to have existed in the past, of any such chain of intermediate 
animal forms as must, on this hypothesis, have existed in order to 
link together the lower forms of life with the vertebrates. 



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12 



THE ORIGIN OF VERTEBRATES 




Fig. 2. 



-Larval Balanoglossus (from the Royal 
Natural History). 



It has been supposed that the Tunicata and the Enteropneusta 
(Balanoglossus) (Fig. 2) are members of this missing chain, and that 

in Amphioxus the ver- 
tebrate approaches in 
organization to these 
low invertebrate forms. 
The tunicates, indeed, 
are looked upon as de- 
generate members of an 
early vertebrate stock, 
which may give help in 
picturing the nature of 
the vertebrate ancestor 
but are not themselves 
in the direct line of 
descent. Balanoglossus 
is supposed to have 
arisen from the Echinodermata, or at all events to have affinities 
with them, so that to fill up the enormous gap between the 
Echinodermata and the Vertebrata on this theory there is absolutely 
nothing living on the earth except Balanoglossus, Bhabdopleura, 
and Cephalodiscus. The characteristics of the vertebrate upon 
which this second theory is based are the notochord, the respiratory 
character of the anterior part of the alimentary canal, and the tubular 
nature of the central nervous system ; it is claimed that in Balano- 
glossus the beginnings of a notochord and a tubular central nervous 
system are to be found, while the respiratory portion of the gut is 
closely comparable to that of Amphioxus. 

The strength of the first theory is essentially based on the com- 
parison of the vertebrate central nervous system with that of the 
segmented invertebrate, annelid or arthropod. In the latter the 
central nervous system is composed of — 

1. The supra-cesophageal ganglia, which give origin to the nerves 
of the eyes and antennules, i.e. to the optic and olfactory nerves, 
for the first pair of antenna are olfactory in function. These are 
connected with the infra-cesophageal ganglia by the oesophageal 
commissures which encircle the oesophagus. 

2. The infra-cesophageal ganglia and the two chains of ventral 
ganglia, which are segmentally-arranged sets of ganglia. Of these, 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 1 3 



each pah* gives rise to the nerves of its own segment, and these 
nerves are not nerves of special sense as are the supra-cesophageal 
nerves, but motor and sensory to the segment ; nerves by the agency 
of which food is taken in and masticated, respiration is effected, and 
the animal moves from place to place. 

In the vertebrate the central nervous system consists of — 
1. The brain proper, from which arise only the olfactory and optic 
nerves. 

Pa 

DORSAL 

Ntuf>«alerte eanai 




Spinal C«rd « Segmental Ntt-vtt 




In/undiM«m VENTRAL 



Anita 



DORSAL 




/ aiopKaya. VENTRAL 

CK« Cmm. 

Fig. 3. — Vertebrate Central Nervous System compared with the Central 
Nervous System and Alimentary Canal of the Arthropod. 

A. Vertebrate central nervous system. S. Inf. Br. y supra-infundibular brain; 
7. Inf. Br. f infra-infundibular brain and cranial segmental nerves; C.Q., corpora 
quadrigemina ; Cb. f cerebellum; C.C., crura cerebri; C.S., corpus striatum; Pn., 
pineal gland. 

B. Invertebrate central nervous system. S. (Es. G., supra-oesophageal ganglia; 
I. (Es, G. t infra-GBsophageal ganglia; (Es. Com., oesophageal commissures. 

2. The region of the mid-brain, medulla oblongata, and spinal 
cord; from these arises a series of nerves segmentally arranged, 
which, as in the invertebrate, gives origin to the nerves governing 
mastication, respiration, and locomotion. 

Further, the vertebrate central nervous system possesses the 
peculiarity, found nowhere else, of being tubular, and the tube is 
of a striking character. In the spinal region it is a small, simple 
canal of uniform calibre, which at the front end dilates to form the 
ventricles of the region of the brain. From that part of this dilated 



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14 THE ORIGIN OF VERTEBRATES 

portion, known as the third ventricle, a narrow tube passes to the 
ventral surface of the brain. This tube is called the infundibulum, 
and, extraordinary to relate, lies just anteriorly to the exits of the 
third cranial or oculomotor nerves; in other words, it marks the 
termination of the series of spinal and cranial segmental nerves. 
Further, on each side of this infundibular tube are lying the two 
thick masses of the crura cerebri, the strands of fibres which connect 
the higher brain-region proper with the lower region of the medulla 
oblongata and spinal cord. Not only, then, are the nerve-masses 
in the two systems exactly comparable, but in the very place where 
the oesophageal tube is found in the invertebrate, the infundibular 
tube exists in the vertebrate, so that if the words infundibular and 
oesophageal are taken to be interchangable, then in every respect 
the two central nervous systems are comparable. The brain proper 
of the vertebrate, with its olfactory and optic nerves, becomes the 
direct descendant of the supra-cesophageal ganglia ; the crura cerebri 
become the oesophageal commissures, and the cranial and spinal 
segmental nerves are respectively the nerves belonging to the infra- 
Gesophageal and ventral chain of ganglia. 

This overwhelmingly strong evidence has always pointed directly 
to the origin of the vertebrate from some form among the segmented 
group of invertebrates, annelid or arthropod, in which the original 
oesophagus had become converted into the infundibulum, and a new 
mouth formed. So far, the position of this school of anatomists was 
extremely sound, for it is impossible to dispute the facts on which 
it is based. Still, however, the fact remained that the gut of the 
vertebrate lies ventrally to the nervous system, while that of the 
invertebrate lies dorsally ; consequently, since the infundibulum was 
in the position of the invertebrate oesophagus, it must originally have 
entered into the gut, and since the vertebrate gut was lying ventrally 
to it, it could only have opened into that gut in the invertebrate stage 
by the shifting of dorsal and ventral surfaces. From this argument 
it followed that the remains of the original mouth into which the in- 
fundibulum, i.e. oesophagus, opened were to be sought for on the dorsal 
side of the vertebrate brain. Here in all vertebrates there are two 
spots where the roof of the brain is very thin, the one in the region of 
the pineal body, and the other constituting the roof of the fourth ven- 
tricle. Both of these places have had their advocates as the position of 
the old mouth, the former being upheld by Owen, the latter by Dohm. 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 1 5 

The discovery that the pineal body was originally an eye, or, 
rather, a pair of eyes, has perhaps more than anything else proved 
the impossibility of accepting this reversal of surfaces as an explana- 
tion of the genesis of the vertebrate from the annelid group. For 
whereas a pair of eyes close to the mid-dorsal line is not only likely 
enough, but is actually found to exist among large numbers of 
arthropods, both living and extinct, a pair of eyes situated close 
to the mid- ventral line near the mouth is not only unheard of in 
nature, but so improbable as to render impossible the theory which 
necessitates such a position. 

Yet this very discovery gives the strongest possible additional 
support to the close identity in the plan of the central nervous 
system of vertebrate and appendiculate. 

A truly paradoxical situation! The very discovery which may 
almost be said to prove the truth of the hypothesis, is the very one 
which has done most to discredit it, because in the minds of its 
authors the only possible solution of the transition from the one 
group to the other was by means of the reversal of surfaces. 

Still, as already said, even if the theory advanced to explain the 
facts be discredited, the facts remain the same ; and still to this day 
an explanation is required as to why such extraordinary resemblances 
should exist between the two nervous systems, unless there is a 
genetic connection between the two groups of animals. An ex- 
planation may still be found, and must be diligently sought for, 
which shall take into account the strong evidence of this relation- 
ship between the two groups, and yet not necessitate any reversal 
of surfaces. It is the object of this book to consider the possibility 
of such an explanation. 

What are the lines of investigation most likely to meet with 
success? Is it possible to lay down any laws of evolution? It 
is instructive to consider the nature of the investigations which 
have led to the two theories just mentioned, for the fundamental 
starting-point is remarkably different in the two cases. The one 
theory is based upon the study of the vertebrate itself, and especially 
of its central nervous system, and its supporters and upholders have 
been and are essentially anatomists, whose chief study is that of 
vertebrate and human anatomy. The other theory is based upon the 
study of the invertebrate, and consists especially of an attempt to 
find in the invertebrate some structure resembling a notochord, such 



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1 6 THE ORIGIN OF VERTEBRATES 

organ being considered by them as the great characteristic of the 
vertebrate; indeed, so much is this the case, that a large number 
of zoologists speak now of Chordata rather than of Vertebrata, and 
in order to emphasize their position follow Bateson, and speak of the 
Tunicata as Uro-chordata, of Amphioxus as Cephalo-chordata, of the 
Enteropneusta as Hemi-chordata, and even of Actinotrocha (to use 
Masterinan's term), as Diplo-chordata. 

The upholders of this theory lay no stress on the nature of the 
central nervous system in vertebrates, they are essentially zoologists 
who have made a special study of the invertebrate rather than of 
the vertebrate. 

Of these two methods of investigating the problem, it must be 
conceded that the former is more likely to give reliable results. 
By putting the vertebrate to the question in every possible way, by 
studying its anatomy and physiology, both gro3S and minute, by 
inquiring into its past history, we can reasonably hope to get a 
clue to its origin, but by no amount of investigation can we tell 
with any certainty what will be its future fate ; we can only guess 
and prophesy in an uncertain and hesitating manner. So it must be 
with any theory of the origin of vertebrates, based on the study of 
one or other invertebrate group. Such theory must partake rather 
of the nature of prophecy than of deduction, and can only be placed 
on a firm basis when it so happens that the investigation of the 
vertebrate points irresistibly to its origin from the same group ; in 
fact, " never prophesy unless you know." 

The first principle, then, I would lay down is this : In order to 
find out the origin of vertebrates, inquire, in the first place, of the 
vertebrate itself. 

Importance of the Central Nervous System. 

Does the history of evolution pick out any particular organ or 
group of organs as more necessary than another for upward progress ? 
If so, it is upon that organ or group of organs that special stress must 
be laid. 

Since Darwin wrote the " Origin of Species," and laid down that 
the law of the ' survival of the fittest ' is the factor upon which evolu- 
tion depends, it has gradually dawned upon the scientific mind that 
' the fittest ' may be produced in two diametrically opposite ways : 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 1 7 

either by progress upwards to a superior form, or by degeneration to 
a lower type of animal. The principle of degeneration as a factor 
in the formation of groups of animals, which are thereby enabled 
to survive, is nowadays universally admitted. The most striking 
example is to be found in the widely distributed group of Tunicata, 
which live, in numbers of instances, a sedentary life upon the rocks, 
have the appearance of very low forms of animal life, propagate 
by budding, have lost all the characteristics of higher forms, and 
yet are considered to be derived from an original vertebrate stock. 
Such degenerate forms remain degenerate, and are never known to 
* * regenerate and again to reach the higher stage of evolution from 
which they arose. Such forms are of considerable interest, but 
cannot help, except negatively, to decide what factor is especially 
important for upward progress. 

At the head of the animal race at the present day stands man, 
and in mankind itself some races are recognized as higher than others. 
Such recognition is given essentially on account of their greater 
brain-power, and without doubt the great characteristic which puts 
man at the head is the development of his central nervous system, 
especially of the region of the brain. Not only is ihis point most 
manifest in distinguishing man from the lower animals, but it applies 
to the latter as well. By the amount of convolution of the brain, 
the amount of grey matter in the cerebral hemispheres, the enlarge- 
ment and increasing complexity of the higher parts of the central 
nervous system, the anthropoid apes are differentiated from the lower 
forms, and the higher mammals from the lower. In the recent work 
of Elliot Smith, and of Edinger, most conclusive proof is given that 
the upward progress in the vertebrate phylum is correlated with the 
increase of brain-power, and the latter writer shows how steady and 
remarkable is the increase in substance and in complexity of the 
brain-region as we pass from the fishes, through the amphibians and 
reptiles, to the birds and mammals. 

The study of the forms which lived on the earth in past age3 con- 
firms and emphasizes this conclusion, for it is most striking to see 
how small is the cranium among the gigantic Dinosaurs ; how in the 
great reptilian age the denizens of the earth were far inferior in brain- 
power to the lords of creation in after-times. 

What applies to the vertebrate phylum applies also to the inver- 
tebrate groups. Here also an upward progress is recognized as we 

c 



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1 8 THE ORIGIN OF VERTEBRATES 

pass from the sponges to the arthropods — a progress which is mani- 
fested, first by the concentration of nervous material to form a central 
nervous system, and then by the increase in substance and complexity 
of that nervous system to form a higher and a higher type, until the 
culmination is reached in the nervous system of the scorpions and 
spiders. No upward progress is possible with degeneration of the 
central nervous system, and in all those cases where a group owes its 
existence to degeneration, the central nervous system takes part in 
the degeneration. 

This law of the paramount importance of the growth of the central 
nervous system for all upward progress in the evolution of animals 
receives confirmation from the study of the development of individuals, 
especially in those cases where a large portion of the life of the 
animal is spent in a larval condition, and then, by a process of trans- 
formation, the larva changes into the adult form. Such cases are 
well known among Arthropoda, the familiar instance being the change 
from the larval caterpillar to the adult imago. Among Vertebrata, 
the change from the tadpole to the frog, from the larval form of 
the lamprey (Ammoccetes) to the adult form (Petromyzon), are well- 
known instances. In all such cases the larva shows signs of having 
attained a certain stage in evolution, and then a remarkable trans- 
formation takes place, with the result that an adult animal emerges, 
whose organization reaches a higher stage of evolution than that of 
the larva. 

This transformation process is characterized by a very great 
destruction of the larval tissues and a subsequent formation of new 
adult tissues. Most extensive is the destruction in the caterpillar 
and in the larval lamprey. But one organ never shares in this process 
of histolysis, and that is the central nervous system ; amidst the 
ruins of the larva it remains, leading and directing the process of 
re-formation. In the Arthropoda, the larval alimentary canal may 
be entirely destroyed and eaten up by phagocytes, but the central 
nervous system not only remains intact but increases in size, and by 
the concentration and cephalization of its infra-oesophageal ganglia 
forms in the adult a central nervous system of a higher type than 
tliat of the larva. 

So, too, in the transformation of the lamprey, there is not the 
slightest trace of any destruction in the central nervous system, but 
simply a development and increase in nervous material, which 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 19 

results in the formation of a brain region more like that of the higher 
vertebrates than exists in Ammoccetes. 

In these cases the development is upward — the adult form is of a 
higher type than that of the larva. It is, however, possible for the 
reverse to occur, so that the individual development leads to degene- 
ration, not to a higher type. Instances are seen in the Tunicata, and 
in various parasitic arthropod forms, such as Lernsea, etc. In these 
cases, the transformation from the larval to the adult form leads to 
degradation, and in this degradation the central nervous system is 
always involved. 

It is perhaps a truism to state that upward progress is necessarily 
accompanied by increased development of the central nervous system ; 
but it is necessary to lay special stress upon the importance of the 
central nervous system in all problems of evolution, because there is, 
in my opinion, a tendency at the present time to ignore this factor to 
too great an extent. 

The law of progress is this — The race is not to the swift, nor to 
the strong, but to the wise. 

This law carries with it the necessary corollary that the imme- 
diate ancestor of the vertebrate must have had a central nervous 
system nearly approaching that of the lowest undegenerated verte- 
brate. Among all the animals living on the earth at the present 
time, the highest invertebrate group, the Arthropoda, possesses 
a central nervous system most closely resembling that of the 
vertebrate. 

The law, then, of the paramount importance of a steady develop- 
ment of the central nervous system for the upward progress of the 
animal kingdom, points directly to the arthropod as the most probable 
ancestor of the vertebrate. 

Evolution of Tissues. 

In the whole scheme of evolution we can recognize, not only an 
upward progress in the organization of the animal as a whole, but 
also a distinct advance in the structure of the tissues composing an 
individual, which accompanies that upward progress. Thus it is 
possible to speak of an evolution of the supporting tissues from the 
simplest form of connective tissue up to cartilage and thence to bone; 
of the contractile tissues, from the simplest contractile protoplasm 



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20 THE GRIG IS OF VERTEBRATES 

to unstriped nmscle, and thence to the highest forms of striated 
muscle ; of the nervous connecting strands, from undifferentiated to 
fine strands, then to thicker, more separated ones, resembling non- 
medullated fibres, and finally to well-differentiated separate fibres, 
each enclosed in a medullated sheath. 

In the connective tissue group, l>one is confined to the vertebrates, 
cartilage is found among invertebrates, and the closest resemblance 
to vertebrate embryonic or parenchymatous cartilage is found in the 
cartilage of Limulus. Also, as Gegenbaur has pointed out, Limulus, 
more than any other invertebrate, possesses a fibrous connective 
tissue resembling that of vertebrates. 

In the muscular group, Biedermann, who has made a special 
study of the physiology of striated muscle, says that among inver- 
tebrates the striated muscle of the arthropod group resembles most 
closely that of the vertebrate. 

In the nervous group the resemblance between the nerve-fibres 
of Limulus and Amraoccctes, both of which are devoid of any marked 
medullary sheath, is very apparent, and Itetzius points out that the 
only evidence of modulation, so characteristic of the vertebrates, is 
found in a species of prawn (Palecmon). In all these cases the 
nearest resemblance to the vertebrate tissues is to be found in the 
arthropod. 

The Evidence of Paleontology. 

Perhaps tho most important of all the clues likely to help in the 
solution of the origin of vertebrates is that afforded by Geology, for 
although the geological record is admittedly so imperfect that we 
can never hoj>e by its means alone to link together the animals at 
present in existence, yet it does undoubtedly point to a sequence in 
the evolution of animal forms, and gives valuable information as to 
the nature of such scqueuee. In different groups of animals there 
are times when the group can be spoken of as having attained its 
most flourishing period. During these geological epochs the dis- 
tribution of the group was universal, the numbers were very great, 
the number of species was at the maximum, and some of them had 
attained a maximal size. Such races were at that time dominant, 
aud the struggle for existence was essentially among members of the 
same gtoup. At the present time the dominant race is man, and the 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 2 1 

struggle for existence is essentially between the members of that 
race, and not between them and any inferior race. 

The effect of such conditions is, as Darwin has pointed out, to 
cause great variation in that group ; in consequence of that variation 
and that dominance the evolution of the next higher group is brought 
about from some member of the dominant group. Thus the present 
age is the outcome of the Tertiary period, a time when giant mammals 
roamed the earth and left as their successors the mammals of the 
present day ; a time of dominance of quadruped mammals ; a time 
of which the period of maximum development is long past, and we 
now see how the dominance of the biped mammal, man, is accom- 
panied by the rapid diminution and approaching extermination of 
the larger mammals. No question can possibly arise as to the im- 
mediate ancestor of the biped mammal ; he undoubtedly arose from 
one of the dominant quadrupedal mammals. 

Passing along to the next evidence of the rocks, we find an age of 
reptiles in the Mesozoic period. Here, again, the number and 
variety is most striking ; here, again, the size is enormous in com- 
parison with that of the present-day members of the group. This 
was the dominant race at the time when the birds and mammals 
first appeared on the earth, and anatomists recognize in these extinct 
reptilian forms two types ; the one bird-like, the other more mamma- 
lian in character. From some members of the former group birds 
are supposed to have been evolved, and mammals from members of 
the other group. There is no question of their origin directly from 
lower fish-like forms ; the time of their appearance on the earth, 
their structure, all point irresistibly to the same conclusion as we 
have arrived at from the consideration of the origin of the biped 
from the quadruped mammal, viz. that birds and mammals arose, in 
consequence of the struggle for existence, from some members of the 
reptilian race which at that time was the dominant one on earth. 

Passing down the geological record, we find that when the reptiles 
first appear in the Carboniferous age there is abundant evidence of 
the existence of numbers of amphibian forms. At this time the 
giant Labyrinthodonts flourished. Here among the swamps and 
marshes of the coal-period the prevalent vertebrate was amphibian 
in structure. Their variety and number were very great, and at that 
period they attained their greatest size. Here, again, from the 
geological record we draw the same conclusion as before, that the 



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< LAST TRLOBITES 



FIRST FISHES 



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FIRST TRILOBITES 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 23 

reptiles arose from the race which was then predominant on the earth 
— the Amphibia. 

Again, another point of great interest is seen here, and that is 
that these Labyrinthodonts, as Huxley has pointed out, possess 
characters which bring them more closely than the amphibians of 
the present day into connection with the fishes; and further, the 
fish-like characters they possessed are those of the Ganoids, the 
Marsipobranchs, the Dipnoans, and the Elasmobranchs, rather than 
of the Teleosteans. 

Now, it is a striking fact that the ancient fishes at the time when 
the amphibians appeared had not reached the teleostean stage. The 
ganoids and elasmobranchs swarmed in the waters of the Devonian 
and Carboniferous times. Dipnoans and marsipobranchs were there, 
too, in all probability, but teleosteans do not appear until the 
Mesozoic period. The very kinds of fish, then, which swarmed in 
the seas at that time, and were the predominant race before the 
Carboniferous epoch, are those to which the amphibians at their first 
appearance show the closest affinity. Here, again, the same law 
appears ; from the predominant race at the time, the next higher 
race arose, and arose by a most striking modification, which was the 
consequence of altering the medium in which it lived. By coming 
out of the water and living on the land, or, rather, being able to live 
partly on land and partly in the water, by the acquisition of air- 
breathing respiratory organs or lungs in addition to, and instead of, 
water-breathing organs or gills, the amphibian not only arose from 
the fish, but made an entirely new departure in the sequence of 
progressive forms. 

This was a most momentous step in the history of evolution— 
one fraught with mighty consequences and full of most important 
suggestions. 

From this time onwards the struggle for existence by which 
upward progress ensued took place on the land, not in the sea, and, 
as has been pointed out, led to the evolution of reptiles from am- 
phibians, birds and quadrupedal mammals from reptiles, and man 
from quadrupeds. In the sea the fishes were left to multiply and 
struggle among themselves, their only opponents being the giant 
cephalopods, which themselves had been evolved from a continual 
succession of the Mollusca. For this reason the struggle for existence 
between the fishes and the higher race evolved from them did not 



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24 THE ORIGIN OF VERTEBRATES 

take place until some members of that higher race took again to the 
water, and so competed with the fish-tribe in their own element. 

Another most important conclusion to be derived from the 
uprising of the Amphibia is that at that time there was no race 
of animals living on the land which had a chance against them. No 
race of land-living animals had been evolved whose organization 
enabled them to compete with and overcome these intruders from 
the sea in the struggle for existence. For this reason that the 
whole land was their own, and no serious competition could arise 
from their congeners, the fish, they took possession of it, and increased 
mightily in size ; losing more and more the habit of going into the 
water, becoming more and more truly terrestrial animals. Hence- 
forth, then, in trying to find out the sequence of evolution, we must 
leave the land and examine the nature of the animals living in the 
sea ; the air-breathing animals which lived on the land in the Upper 
Silurian and Devonian times cannot have reached a stage of organi- 
zation comparable with that of the fishes, seeing how easily the 
amphibians became dominant. 

We arrive, then, at the conclusion that the ancestors of the fishes 
must have lived in the sea, and applying still the same principles 
that have held good up to this time, the ancestors of the fishes must 
have arisen from some member of the race predominant at the time 
when they first appeared, and also the earliest fishes must have much 
more closely resembled the ancestral form than those found in later 
times or at the present day. 

What, then, is the record of the rocks at the time of the first 
appearance of fish-like forms ? What kind of fishes were they, and 
what was the predominant race at the time ? 

We have now reached the Upper Silurian and Lower Devonian 
times, and most instructive and suggestive is the revelation of the 
rocks. Here, when the first vertebrates appeared, the sea was peopled 
with corals, brachiopods, early forms of cephalopods, and other in- 
vertebrates ; but, above all, with the great tribe of trilobites (Fig. 6) 
and their successors. From the trilobites arose, as evidenced by 
their larval form, the king-crab group, called the Xiphosura (Fig. 5). 
Closely connected with them, and forming intermediate stages 
between trilobites and king-crabs, numerous forms have been dis- 
covered, known as Belinurus, Prestwichia, Hemiaspis, Bunodes, etc. 
(Fig. 5 and Fig. 12). From them also arose the most striking group 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 25 

of animals which existed at this period — the giant sea-scorpions, or 
Gigantostraca. This group was closely associated with the king- 
crabs, and the two groups together are classified under the title 
Merostomata. 

The appearance of these sea-scorpions is given in Figs. 7 and 8, 
representing Stylonurus, Slimonia, Pterygotus, Eurypterus. They 




Fig. 5 (from H. Woodward). — 1. Limulus polyphcmus (dorsal aspoct). 2. Limulus, 
young, in trilobitc stage. 8. Prestwichia rotundata. i. Prestwichia Dirtwelli. 
5. llemiaspis limuloides. 6. Psciidoniscus aculeatus. 

must have been in those days the tyrants of the deep, for specimens 
of Pterygotus have been found over six feet in length. 

At this time, then, by every criterion hitherto used, by the 
multitude of species, by the size of individual species, which at this 
period reached the maximum, by their subsequent decay and final 
extinction, we must conclude that these forms were in their zenith, 
that the predominant race at this time was to be found in this group 
of arthropods. Just previously, the sea swarmed with trilobites, and 
right into the period when the Gigantostraca flourished, the tril obites 



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26 



THE ORIGIN OF VERTEBRATES 



are still found of countless forms, of great difference in size. The 
whole period may be spoken of as the great trilobite age, just as the 
Tertiary times form the mammalian age, the Mesozoic times the 
reptilian age, etc. From the trilobites the Gigantostraca and 
Xiphosura arose, as evidenced by the embryology of Limulus, and, 
therefore, in the term trilobite age would be included the whole of 
those peculiar forms which are classified by the names Trilobita, 





Fig. 6.— A Tbilobite (Dalma- 
tite*) (after Pictet). Dorsal 



Fig. 7. — Euryplerus remijpes (after 
Nikskowski). Dorsal view. 



* 



Gigantostraca, Xiphosura, etc. Of all these the only member alive 
at the present time is Limulus, or the King-Crab. 

As, however, the term ' trilobite ' does not include the members 
of the king-crab or sea-scorpion groups, it is advisable to use some 
other term to represent the whole group. They cannot be called 
crustaceans or arachnids, for in all probability they gave origin to 
both ; the nearest approach to the Trilobite stage of development at 
the present time is to be found perhaps in Branchipus (Fig. 10) and 
Apus (Fig. 9), just as the nearest approach to the Eurypterid 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 2 J 

form is Limulus. Crustaceans such as crabs and lobsters are of 
much later origin, and do not occur in any quantity until the late 




Fig. 8.— A, Pterygotus Osiliensis (from Schmidt). B, Stylonurus Logani (from 
Woodward). C, Slimonia acuminata (from Woodward). 

Mesozoic period. The earliest found, a kind of prawn, occurs in the 
Carboniferous age. 

Korschelt and Heider have accordingly suggested the name 
Palccostraca for this whole group, and Protostraca for the still earlier 



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28 



THE ORIGIN OF VERTEBRATES 



arthropod-like animals which gave origin to the trilobites themselves. 
This name I shall adopt, and speak, therefore, of the Palceostraca as 

the dominant race at the time when 
vertebrates first appeared. 

If, then, there is no break in the 
law of evolution here, the race which 
was predominant at the time when 
the vertebrate first appeared must 
have been that from which the first 
fishes arose, and these fishes must 
have resembled, not the crustacean 
proper, or the arachnid proper, but a 
member of the palseostracan group. 
Moreover, just as the Labyrinthodonts 
show special affinities to the fishes 
which were then living, so we should 
expect that the forms of the earliest 
fish would resemble the arthropodan 
type dominant at the time more 
closely than the fish of a later era. 

At first sight it seems too great 

an absurdity even to imagine the 

possibility of any genetic connection between a fish and an arthropod, 

for to the mind's eye there arises immediately the picture of a 

salmon or a shark and a lobster or a spider. So different in appear- 




Fig. 9. — Apus (from the Royal 
Natural History). Dorsal view. 




Fig. 10.— Branchipus stagnalis. (From Claus.) 

ance are the two groups of animals, so different their methods of 
locomotion, that it is apparently only an inmate of a lunatic asylum 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 29 

who could possibly suggest such a connection. Much more likely 
is it that a fish-like form should have been developed out of a smooth, 
wriggling, worm-like animal, and it is therefore to the annelids that 
the upholders of the theory of the reversal of surfaces look for the 
ancestor of the vertebrate. 

We must endeavour to dismiss from our imagination such forms 
as the salmon and shark as representatives of the fish-tribe, and the 
lobster and spider of the arthropods, and try to picture the kind of 
animals living in the seas in the early Devonian and Upper Silurian 
times, and then we find, to our surprise, that instead of the contrast 
between fishes and arthropods being so striking as to make any 
comparison between the two seem an absurdity, the difficulty in the 
last century, and even now, is to decide in many cases whether a 
fossil is an arthropod or a fish. 

I have shown what kind of animal the palaeostracan was like. 
What information is there of the nature of the earliest vertebrate ? 

The most ancient fishes hitherto discovered have been classified 
by Lankester and Smith Woodward into the three orders, Hetero- 
straci, Osteostraci, and Antiarcha. Of these the Heterostraci contain 
the genera Pteraspis and Cyathaspis, and are the very earliest 
vertebrates yet discovered, being found in the Lower Silurian. The 
Osteostraci are divided into the Cephalaspida), Tremataspidae, etc., 
and are found in the Upper Silurian and Devonian beds. The 
Antiarcha, comprising Pterichthys and Bothriolepis, belong to the 
Devonian and are not found in Silurian deposits. This, then, is the 
order of their appearance — Pteraspis, Cephalaspis, and Pterichthys. 

In none of these families is there any resemblance to an ordinary 
fish. In no case is there any sign of vertebrae or of jaws. They, like 
the lampreys, were all agnathostomatous. Strange indeed is their 
appearance, and it is no wonder that there should have been a 
difficulty in deciding whether they were fish or arthropod. Their great 
characteristic is their buckler- plated cephalic shield, especially con- 
spicuous on the dorsal side of the head. Figs. 11, 14, 15, 16, give 
the dorsal shields of Pteraspis, Auchenaspis, Pterichthys, and 
Bothriolepis. 

In 1904, Drevermann discovered a mass of Pteraspis Dunensis 
embedded in a single stone, showing the same kind of head-shield 
as P. rostrata, but the rostrum was longer and the spine at the 
extremity of the head-shield much longer and more conspicuous. 



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3° 



THE ORIGIN OF VERTEBRATES 




Fia. 11.— Pteraspis dunensis (from Drevermann). Dorsal view of body and spine 
on the right side. Head-end, showing long rostrum on the left side. 





Fig. 12.— Bunodes lunula. 
Schmidt.) 



(From Fig. 13. — Auchcnaspis (Thyestes) verru- 

cosus, natural size. (From Woodward.) 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 3 1 

The whole shape of the animal as seen in this photograph recalls the 
shape of a Hemiaspid rather than of a fish. It is, then, natural 
enough for the earlier observers to have looked upon such a fossil as 
related to an arthropod rather than a fish. 

In Figs. 12 and 13 I have placed side by side two Silurian fossils 
which are found in the same geological horizon. They are both life 
size and possess a general similarity of appearance, yet the one is a 




Fig. 14.— Dobsal Head-shield op Thy 
esUs (Auchenaspis) verrucosus. (From 
Kohon.) 

Fro., narial opening ; l.e., lateral eyes ; gl. t 
glabellum or plate over brain ; Occ, oc- 
cipital region. 




Fio. 15.—Ptcricthys. 



Cephalaspidian fish known by the name of Auchenaspis or Thycstes 
verrucosa, the other a Palreostracan called Bunodes lunula. 

In a later chapter I propose to discuss the peculiarities and the 
nature of the head-shields of these earliest fishes, in connection with 
the question of the affinities of the animals which bore them. At 
this point of my argument I want simply to draw attention to the 
undoubted fact of the striking similarity in appearance between the 



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32 



THE ORIGIN OF VERTEBRATES 



earliest fishes and members of the Palseostraca, the dominant race of 
arthropods which swarmed in the sea at the time : a similarity which 
could never have been suspected by any amount of investigation 




Fio. 16.— Bothriolepis. (After Patten.) 
An., position of anus. 

among living forms, but is immediately revealed when the ages 
themselves are questioned. 

I have not reproduced any of the attempted restorations of these 
old forms, as usually given in the text-books, because all such restora- 
tions possess a large element of fancy, due to the personal bias of the 
observer. I have put in Eohon's idea of the general shape of Tre- 
mataspis (Fig. 17) in order to draw attention to the lamprey-like 
appearance of the fish according to his researches (cf. Fig. 18). 




Fig. 17.— Restoration of Trcmataspis. (After Rohon, slightly modified.) 




4 



Fio. 18.— Ammocates. 



The argument, then, from geology, like that from comparative 

tomy and from the consideration of the importance of the central 

9 system in the upward development of the animal race, not 

)ints directly to the arthropod group as the ancestor of the 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 33 

vertebrate, but also to a distinct ancient type of arthropod, the 
Palaeostracan, the only living example of which is the King-Crab or 
Limulus; while the nearest approach to the trilobite group among 
living arthropods are Branchipus and Apus. It follows, therefore, that 
for the following up of this clue, Limulus especially must be taken into 
consideration, while Branchipus and Apus are always to be kept in mind. 

Ammoccetes rather than Amphioxus is the Best Subject for 

Investigation. 

It is not, however, Limulus that must be investigated in the first 
instance, but the vertebrate itself; for it can never be insisted on too 
often that in the vertebrate itself its past history will be found, but 
that Limulus cannot reveal the future of its race. What vertebrate 
must be chosen for investigation? Eeasons have been given why 
our attention should be fixed upon the king-crab rather than on the 
lobster on the invertebrate side; what is the most likely animal on 
the vertebrate side ? 

From the evidence already given it is manifest that the earliest 
mammal belonged to the lowest group of mammals ; that the birds 
on their first appearance presented reptilian characteristics, that the 
earliest reptiles belonged to a low type of reptile, that the amphibians 
at their first appearance were nearer in type to the fishes than were 
the later forms. As each of these groups advances in number and 
power, specialization takes place in it, and the latest developed 
members become further and further removed in type from the 
earliest. So also it must have been with the origin of fishes : here 
too, in the quest for information as to the structure and nature of 
the first-formed fishes, we must look to the lowest rather than to 
the highest living members of the group. 

The lowest fish-like animal at present living is Amphioxus, and 
on this ground it is argued that the original vertebrate must have 
approached in organization to that of Amphioxus; it is upon the 
comparison between the structure of Amphioxus and that of Balano- 
glossus, that the theory of the origin of vertebrates from forms like 
the latter animal is based. For my own part, I think that in the 
first instance, at all events, Amphioxus should be put on one side, 
although of course its structure must always be kept in mind, for 
the following reasons : — 

D 



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34 THE ORIGIN OF VERTEBRATES 

Amphioxus, like the tunicates, does not possess the character- 
istics of other vertebrates. In all vertebrates above these forms 
the great characteristic is a well-defined brain-region from which 
arise nerves to organs of special sense, the eyes and nose. In 
Amphioxus no eyes exist, for the pigmented spot at the anterior 
extremity of the brain-region is no eye but only a mass of pig- 
ment, and the so-called olfactory pit is a very rudimentary and 
inferior organ of smell. In connection with the nearly complete 
absence of these two most important sense-organs, the most im- 
portant part of the central nervous system, the region corresponding 
to the cerebral hemispheres, is also nearly completely absent. 

Now, the history of the evolution of the central nervous system in 
the animal race points directly to its formation as a concentrated 
mass of nervous material at the anterior extremity of the body, in 
consequence of the formation of special olfactory and visual organs 
at that extremity. As already stated, the concentration of nervous 
material around the mouth as an oral ring was its beginning. In 
connection with this there arose special sense-organs for the guidance 
of the animal to its food which took the form of olfactory and optic 
organs. With the shifting from the radial to the elongated form 
these sense-organs remained at the anterior or mouth-end of the 
animal, and owing to their immense importance in the struggle for 
existence, that part of the central nervous system with which they 
were connected developed more than any other part, became the 
leader to which the rest of the nervous system was subservient, and 
from that time onwards the development of the brain-region was 
inevitably associated with the upward progress of animal life. 

To those who believe in Evolution and the Darwinian theory of 
the survival of the fittest, it is simply inconceivable that a soft-bodied 
animal living in the mud, blind, with a rudimentary brain and rudi- 
mentary olfactory organs, such as is postulated when we think of 
Balanoglossus and Amphioxus, should hold its own and come victorious 
out of the struggle for existence at a time when the sea was peopled 
with powerful predaceous scorpion- and crab-like armour-plated 
animals possessing a well-developed brain, good eyes and olfactory 
organs, and powerful means of locomotion. Wherever in the scale of 
animal development Amphioxus may ultimately be placed, it cannot 
be looked upon as the type of the earliest formed fishes such as 
appeared in Silurian times. 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 35 

The next lowest group of living fishes is the Marsipobranchii which 
include the lampreys and hag-fishes. To these naturally we must turn 
for a clue as to the organization of the earliest fish, for here we find 
all the characteristics of the vertebrates represented : a well-formed 
brain-region, well-developed eyes and nose, cranial nerves directly 
comparable with those of other vertebrates, and even the commence- 
ment of vertebra. 

Among these forms the lamprey is by far the best for investiga- 
tion, not only because it is easily obtainable in large quantities, but 
especially because it passes a large portion of its existence in a larval 
condition, from which it emerges into the adult state by a wonderful 
process of transformation, comparable in extent with the transforma- 
tion of the larval caterpillar into the adult imago. So long does the 
lamprey live in this free larval condition, and so different is it in 
the adult stage, that the older anatomists considered that the two 
states were really different species, and gave the name of Am- 
moccetc* branchialis to the larval stage, while the adult form was 
called Petromyzon planeri, or Petromyzon fltiviatilis. 

This long-continued free-living existence in the larval or Am- 
moccetes stage makes the lamprey, more than any other type of 
lowly organized fish, invaluable for the present investigation, for 
throughout the animal kingdom it is recognized that the larval 
form approaches nearer to the ancestral type than the adult form, 
whether the latter is progressive or degenerate. Not only are the 
tissues formed during the stages which are passed through in a 
free-living larval form, serviceable tissues comparable to those 
of adult life, but also these stages proceed at so much slower a rate 
than do those in the embryo in utero or in the egg, as to make 
the larval form much more suitable than the embryo for the investi- 
gation of ancestral problems. It is true enough that the free life of 
the larva may bring about special adaptations which are not of an 
ancestral character, as may also occur during the life of the adult ; 
but the evidence is very strong that although some of the peculi- 
arities of the larva may be due to such ccenogenetic factors, yet on 
the whole many of them are due to ancestral characters, which dis- 
appear when transformation takes place, and are not found in the 
adult. 

Thus if it be supposed that the amphibian arose from the fish, 
the tadpole presents more resemblance to the fish than the frog. If 



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36 THE ORIGIN OF VERTEBRATES 

it be supposed that the arthropod arose from the segmented worm, 
the caterpillar bears out the suggestion better than the adult imago. 
If it be supposed that the tunicate arose from a stock allied to the 
vertebrate, it is because of the peculiarities of the Lirva that such a 
supposition is entertained. So, too, if it be supposed that the fish 
arose from a member of the arthropod group, the larval form of the 
fish is most likely to give decisive information on the point. 

For all these reasons the lowest form of fish to be investigated, 
in the hopes of finding out the nature of the earliest formed fish, is 
not Amphioxus, but Ammocoetes, the larval form of the lamprey — a 
form which, as I hope to satisfy my reader after perusal of subse- 
quent pages, more nearly resembles the ancient Cephalaspidian fishes 
than any other living vertebrates. 

Comparison of Central Nervous Systems of Vertebrate and 
Arthropod without Eeversal of Surfaces. 

So far different lines of investigation all point to the origin of the 
vertebrate from arthropods, the group of arthropods in question being 
now extinct, the nearest living representative being Limulus ; also to 
the fact that of the two theories of the origin of vertebrates, that 
one which is based on the resemblance between the central nervous 
systems of the Vertebrata and the Appendiculata (Arthropoda and 
Annelida) is more in accordance with this evidence than the other, 
which is based mainly on the supposed possession of a notochord 
among certain animals. 

How is it, then, that this theory has been discredited and lost 
ground ? Simply, I imagine, because it was thought to necessitate 
the turning over of the animal. Let us, then, again look at the 
nervous system of the vertebrate, and see whether there is any such 
necessity. 

As previously mentioned, the comparison of the two central 
nervous systems showed such close resemblances as to force those 
anatomists who supported this theory to the conclusion that the 
infundibular tube was in the position of the original oesophagus ; 
they therefore looked for the remains of a mouth opening in the 
dorsal roof of the brain, but did not attempt to explain the extra- 
ordinary fact that the infundibular tube is only a ventral offshoot 
from the tube of the central nervous system. Yet this latter tube 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 37 

is one, if not the most striking, of the peculiarities which distinguish 
the vertebrate ; a tubular central nervous system such as that of the 
vertebrate is totally unlike any other nervous system, and the very 
fact that the two nervous systems of the vertebrate and arthropod 
are so similar in their nervous arrangements, makes it still more 
extraordinary that the nervous system should be grouped round a 
tube in the one case and not in the other. 

Now, in the arthropod the oesophagus leads directly into the 
stomach, which is situated in the head- region, and from this a straight 
intestine passes directly along the length of the body to the anus, 
where it terminates. The relations of mouth, oesophagus, alimentary 
canal, and nervous system in these animals are represented in the 
diagram (Fig. 3). 

Any tube, therefore, such as that of the infundibulum, which 
would represent the oesophagus of such an animal, must have opened 
into the mouth on the ventral side, and into the stomach on the 
dorsal side, and the lining epithelium of such an oesophagus must 
have been continuous with that of the stomach, and so of the whole 
intestinal tract. 

Supposing, then, the animal is not turned over, but that the dorsal 
side still remains dorsal and ventral ventral, then the original mouth- 
opening of the oesophagus must be looked for on the ventral surface 
of the vertebrate brain in the region of the pituitary body or hypo- 
physis, and on the dorsal side the tube representing the oesophagus 
must be continuous with a large cephalically dilated tube, which 
ought to pass into a small canal, to run along the length of the body 
and terminate in the anus. 

This is exactly what is found in the vertebrate, for the infun- 
dibular tube passes into the third ventricle of the brain, which forms, 
with the other ventricles of the brain, the large dilated cephalic 
portion of the so-called nerve tube, and at the junction of the medulla 
oblongata and spinal cord, this dilated anterior part passes into the 
small, straight, central canal of the spinal cord, which in the embryo 
terminates in the anus by way of the neurenteric canal. If the 
animal is regarded as not having been turned over, then the con- 
clusion that the infundibulum was the original oesophagus leads 
immediately to the further conclusion that the ventricles of the verte- 
brate brain represent the original cephalic stomach, and the central 
canal of the spinal cord the straight intestine of the arthropod ancestor. 



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38 THE ORIGIN OF VERTEBRATES 

For the first time a logical, straightforward explanation is thus 
given of the peculiarities of the tube of the central nervous system, 
with its extraordinary termination in the anus in the embryo, its 
smallness in the spinal cord, its largeness in the brain region, and its 
offshoot to the ventral side of the brain as the infundibular channel. 
It is so clear that, if the infundibular tube be looked on as the old 
cesophagus, then its lining epithelium is the lining of that oesophagus ; 
and the fact that this lining epithelium is continuous with that of 
the third ventricle, and so with the lining of the whole nerve-tube, 
must be taken into account and not entirely ignored as has hitherto 
been the case. If, then, we look at the central nervous system of 
the vertebrate in the light of the central nervous system of the 
arthropod without turning the animal over, we are led immediately 
to the conclusion that what has hitherto been called the vertebrate 
nervous system is in reality composed of two parts, viz. a nervous 
part comparable in all respects with that of the arthropod ancestor, 
which has grown over and included into itself, to a greater or less 
extent, a tubular part comparable in all respects with the alimentary 
canal of the aforesaid ancestor. If this conclusion is correct, it is 
entirely wrong to speak of the vertebrate central nervous system as 
being tubular, for the tube does not belong to the nervous system, 
but was originally a simple epithelial tube, such as characterizes the 
cesophagus, cephalic stomach, and straight intestine of the arthropod. 

Here, then, is the crux of the position — either the so-called 
nervous tube of the vertebrate is composed of two separate factors, 
consisting of a true non-tubular nervous system and a non-nervous 
epithelial tube, these two elements having become closely connected 
together ; or it is composed of one factor, an epithelial tube which 
constitutes the nervous system, its elements being all nervous 
elements. 

If this latter hypothesis be accepted, then it is necessary to 
explain why parts of that tube, such as the roof of the fourth 
ventricle, the choroid plexuses of the various ventricles, which are 
parts of the original roof inserted into the ventricles, are not com- 
posed of nervous material, but form simple single-layered epithelial 
sheets, which by no possibility can be included among functional 
nervous structures. The upholders of this hypothesis can only 
explain the nature of these thin epithelial parts of the nervous tube 
in one of two ways ; either the tube was originally formed of nervous 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEAf 39 

material throughout, and for some reason parts of it have lost their 
nervous function and thinned down; or else these thin epithelial 
parts are on their way to become nervous material, are still in an 
embryonic condition, and are of the nature of epiblast-epitheliuin, 
from which the central nervous system originally arose. 

The first explanation is said to be supported by embryology, for 
at first the nerve-tube is formed in a uniform manner, and then 
later, parts of the roof appear to thin out and so form the thin epi- 
thelial parts. If this were the right explanation, then it ought to 
be found that in the lowest vertebrates there is greater evidence of 
a uniformly nervous tube than in the higher members of the group : 
while conversely, if, on the contrary, as we descend the vertebrate 
phylum, it is found that more and more of the tube presents the 
appearance of a single layer of epithelium, and the nervous material 
is limited more and more to certain parts of that tube, then the 
evidence is strong that the tubular character of the central nervous 
system is not due to an original nervous tube, but to a non-nervous 
epithelial tube with which the original nervous system has become 
closely connected. 

The comparison of the brain region of the different groups of 
vertebrates (Fig. 19) is most instructive, for it demonstrates in the 
most conclusive manner how the roof of the nervous tube in that 
region loses more and more its nervous character, and takes on the 
appearance of a simple epithelial tube, as we descend lower and 
' lower ; until at last, in the brain of Ammocoetes, as represented in 
the figures, the whole of the brain- roof, from the region of the 
pineal eye to the commencement of the spinal cord, is composed of 
fold upon fold of a thin epithelial membrane forming an epithelial 
bag, which is constricted in only one place, where the fourth cranial 
nerve crosses over it. 

Further, the brain of Ammocoetes (Fig. 20) shows clearly not only 
that it is composed of two parts, an epithelial tube and a nervous 
system, but also that the nerve-masses are arranged in the same 
relative position with respect to this tube as are the nerve-masses in 
the invertebrate with respect to the cephalic stomach and oesophagus. 
This evidence is so striking, so conclusive, that it is impossible to 
resist the conclusion that the tube did not originate as part of the 
central nervous system, but was originally independent of the central 
nervous system, and has been invaded by it. 



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4Q 



THE ORIGIN OF VERTEBRATES 



MAMMALIA. 



REPTILIA. 



AMPHIBIA 



TELE08TEA 




AMM0CCETE8. 



Fig. 19.- Comparison of Vertebrate Brains. 

CB., cerebellum ; VT., pituitary body ; PN. t pineal body ; C. STli., corpus striatum ; 
G.H.H., rigbt ganglion habouuke. I., olfactory; II. , optic nerves. 



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GHR 



Fig. 20. — Brain of 
Ammoccetes. 

A, dorsal view ; B, late- 
ral view; C, ventral 
view. 



C.E.Ii.y cerobral hemi- 
spheres ; G.U.R., 
right ganglion habc- 
nulte ; PN., right 
pineal eye ; C7/ s , 
CH 3f choroid plex- 
uses ; I.-XIL cra- 
nial nerves ; C.P., 
Conus 2 x>s i" comtt ^a- 
suralis. 



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42 THE ORIGIN OF VERTEBRATES 

The second explanation is hardly worth serious consideration, for 
it supposes that the nervous system, for no possible reason, was laid 
down in its most important parts — the brain-region — as an epithelial 
tube with latent potential nervous functions; that even up to the 
highest vertebrate yet evolved these nervous functions are still in 
abeyance over the whole of the choroid plexuses and the roof of the 
fourth ventricle. Further, it supposes that this prophetic epithelial 
tube originally developed into true nervous material only in certain 
parts, and that these parts, curiously enough, formed a nervous 
system absolutely comparable to that of the arthropod, while the 
dormant prophetic epithelial part was formed so as just to mimic, 
in relation to the nervous part, the alimentary canal of that same 
arthropod. 

The mere facts of the case are sufficient to show the glaring 
absurdity of such an explanation. This is not the way Nature works ; 
it is not consistent with natural selection to suppose that in a low 
form nervous material can be laid down as non-nervous epithelial 
material in order to provide in some future ages for the great increase 
in the nervous system. 

Every method of investigation points to the same conclusion, 
whether the method is embryological, anatomical, or pathological. 

First, take the embryological evidence. On the ground that the 
individual development reproduces to a certain extent the phylo- 
genetic development, the peculiarities of the formation of the central 
nervous system in the vertebrate embryo ought to receive an appro- 
priate explanation in any theory of phylogenetic development. 
Hitherto such explanation has been totally lacking ; any suggestion 
of the manner in which a tubular nervous system may have been 
formed takes no account whatever of the differences between different 
parts of the tube; its dilated cephalic end with its infundibular 
projection ventrally, its small straight spinal part, and its termination 
in the anus. My theory, on the other hand, is in perfect harmony 
with the embryological history, and explains it point by point. 

From the very first origin of the central nervous system there 
is evidence of two structures — the one nervous, and the other an 
epithelial surface-layer which ultimately forms a tube; this was 
first described by Scott in Petromyzon, and later by Assheton in the 
frog. In the latter case the external epithelial layer is pigmented, 
while the underlying nervous layer contains no pigment ; a marked 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 43 

and conspicuous demarcation exists, therefore, between the two layers 
from the very beginning, and it is easy to trace the subsequent fate 
of the two layers owing to this difference of pigmentation. The pig- 
mented cells form the lining cells of the central canal, and becoming 
elongated, stretch out between the cells of the nervous layer ; while 
the latter, on their side, invade and press between the pigmented 
cells. In this case, owing to the pigmentation of the epithelial layer, 
embryology points out in the clearest possible manner how the 
central nervous system of the vertebrate is composed of two struc- 
tures — an epithelial non-nervous tube, on the outside of which the 
central nervous system was originally grouped; how, as develop- 
ment proceeds, the elements of these two structures invade each 
other, until at last they become so involved together as to give rise 
to the conception that we are dealing with one single nerve tube. 
It is impossible for embryology to give a clearer clue to the past 
history than it does in this case, for it actually shows, step by step, 
how the amalgamation between the central nervous system and the 
old alimentary canal took place. 

Further, consider the shape of the tube when it is first formed, 
how extraordinary and significant that is. It consists of a simple 
dilated anterior end leading into a straight tube, the lumen of which 
is much larger than that of the ultimate spinal canal, and terminates 
by way of the neurenteric canal in the anus. 

Why should the tube take this peculiar shape at its first forma- 
tion ? No explanation is given or suggested in any text-book of 
embryology, and yet it is so natural, so simple : it is simply the shape 
of the invertebrate alimentary canal with its cephalic stomach and 
straight intestine ending in the anus. Again embryology indicates 
most unmistakably the past history of the race. How are the 
nervous elements grouped round this tube when it is first formed ? 
Here embryology shows that a striking difference exists between the 
part of the tube which forms the spinal cord and the dilated cephalic 
part. Fig. 21, A (2), represents the relation between the nervous 
masses and the epithelial tube in the first instance. At this stage 
the nervous material in the spinal cord lies laterally and ventrally 
to this tube, and at a very early stage the white anterior commissure 
is formed, joining together these two lateral masses ; as yet there is 
no sign of any posterior fissure, the tube with its open lumen extends 
right to the dorsal surface. 



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44 



THE ORIGIN OF VERTEBRATES 



The interpretation of this stage is that in the invertebrate ancestor 
the nerve-inasses were situated laterally and ventrally to the 
epithelial tube, and were connected together by commissures on the 
ventral side of the tube (Fig. 21, A (1)) ; in other words, the chain of 
ventral ganglia and their transverse commissures lying just ventrally 
to the intestine, which are so characteristic of the arthropod nervous 
system, is represented at tliis stage. 

Subsequently, by the growth dorsalwards of nervous material to 
form the posterior columns, the original epithelial tube is compressed 
dorsally and laterally to such an extent that those parts lose all signs 
of lumen, the one becoming the posterior fissure and the others the 

3 ^ 

A i 2 






Fig. 21. — A, Method of Formation of the Vertebrate Spinal Cord from the 
Ventral Chain of Ganglia and the Intestine of an Arthropod, repre- 
sented in 1 ; B, Method of Formation of the Vertebrate Medulla 
Oblongata from the Infra-o:sophageal Ganglia and the Cephalic 
Stomach of an Arthropod. 

substantia gel a tiuosa Rolandi on each side. The original tube is thus 
reduced to a small canal formed by its ventral portion only (Fig. 21, 
A (3)). In this way the spinal cord is formed, and the walls of the 
original epithelial tube are finally visible only as the lining of the 
central canal (Fig. 21, A (4)). 

When we pass to the brain-region, to the anterior dilated 
portion of the tube, embryology tells a different story. Here, as in 
the spinal cord, the nervous masses are grouped at first laterally and 
ventrally to the epithelial tube, as is seen in Fig. 21, B (2), but owing 
to the large size of its lumen here, the nervous material is not 
able to enclose it completely, as in the case of the spinal cord ; 



s~ 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 45 

consequently there is no posterior fissure formed ; but, on the contrary, 
the dorsal roof, not enclosed by the nerve-masses, remains epithelial, 
and so forms the membranous roof of the fourth ventricle and of the 
other ventricles of the brain (Fig. 21, B (3)). In the higher animals, 
owing to the development of the cerebrum and cerebellum, this 
membranous roof becomes pushed into the larger brain cavity, and 
thus forms the choroid plexuses of the third and lateral ventricles. 
In the lower vertebrates, as in Ammoccetes and the Dipnoi, it still 
remains as a dorsal epithelial roof and forms a most striking 
characteristic of such brains. 

In this part of the nervous system, then, the nervous material is 
all grouped in its original position on the ventral side of the tube ; 
and yet it is the same nervous material as that of the spinal cord, 
all the elements are there, giving origin here to the segmental cranial 
nerves just as lower down they give rise to the segmental spinal 
nerves, connecting together the separate segments each with the other 
and all with the higher brain-centres — the supra-infundibular centres 
— just as they do in the spinal region. 

Why should there be this striking difference between the 
formation of the infra-infundibular region of the brain and that of 
the spinal cord ? Do the advocates of the origin of vertebrates from 
Balanoglossus give the slightest reason for it ? They claim that their 
view also provides a tubular nervous system for the vertebrate, but 
give not the slightest sign or indication as to why the nervous 
material should be grouped entirely on the ventral side of an 
epithelial tube in the infra-infundibular region and yet surround 
it in the spinal cord region. And the explanation is so natural, 
so simple : embryology does its very best to tell us the past history 
of the race, if only we look at it the right way. 

The infra-infundibular nervous mass is naturally confined to the 
ventral side of the epithelial tube, because it represents the infra- 
oesophageal ganglia, situated as they are on the ventral side of the 
cephalic stomach, and, owing to the size of the stomach, they could 
not enclose it by dorsal growth, as they do in the case of the forma- 
tion of the spinal cord (Fig. 21, B (1)). Still these nervous masses 
have grown dorsal wards, have commenced to involve the walls of 
the cephalic stomach even in the lowest vertebrate, as is seen in 
Ammoccetes, in which animal a ventral portion of the epithelial 
bag has been evidently compressed and its lumen finally obliterated 



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4 6 



THE ORIGIN OF VERTEBRATES 



by the growth of the nerve-masses on each side of it. Throughout 
the whole vertebrate kingdom this obliterated portion still leaves 

its mark as the rajyhe or seam, 
which is so characteristic of 
the infra-infundibular portion 
of the brain. 

Here, again, it is seen how 
simple is the explanation of a 
peculiarity which has always 
puzzled anatomists —why 
should there be this seam in 
the infra-infundibular portion 
of the brain and not in the 
supra-infundibular or in the 
spinal cord ? The correspond- 
ing compression in the upper 
brain-region forms the lateral 
ventricles, as is seen in the 
accompanying figure of the 
brain of Ammoccetes (Fig. 22). 
In yet another instance it is 
seen how markedly the nervous 
masses are arranged in the 

T „ . same position with respect to 

Cr., membranous cranium ; I, olfactory * 

nerves; l.v., lateral ventricles; gl. , gl&n- the Central tube as are the 

dular tissue which fills up the cranial nerve ganglia with respect to 

cavity - the intestinal tube in the case 

of the invertebrate. Thus in birds a portion of the spinal cord 

in the lumbo-sacral region presents a very different appearance 

from the rest of the cord ; it is 
known as the rhomboidal sinus, 
and a section of the cord of an 
adult pigeon across this region is 
given in Fig. 23. As is seen, the 
nervous portions are entirely con- 
Pig. 23.— Skction through Rhomboidal fi ne d to two masses connected 

together by the white anterior 
commissures which are situated laterally and ventrally to a 
median gelatinous mass ; the small central canal is visible and 




Fig. 22.— Horizontal Section through 
the Brain op Ammocxetes. 




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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 47 

the whole dorsal area of the cord is taken up by a peculiar non- 
nervous wedge-shaped mass of tissue. At its first formation this 
portion of the cord is formed exactly in the same manner as the rest 
of the cord ; instead, however, of the nervous material invading the 
dorsal part of the tube to form the posterior fissure, it has been from 
some cause unable to do so, the walls of the original non-nervous 
tube have become thickened dorsally, been transformed into this 
peculiar tissue, and so caused the peculiar appearance of the cord 
here. The nervous parts have not suffered in their development ; 
the mechanism for walking in the bird is as well developed as in 
any other animal ; their position only is different, for they still retain 
the original ventro-lateral position, but the non-nervous tube, the 
remains of the old intestine, has undergone a peculiar gelatinous 
degeneration just where it has remained free from invasion by the 
nervous tissue. 

Throughout the whole of that part of the nervous system which 
gives origin to the cranial and spinal segmental nerves, the evidence 
is absolutely uniform that the nervous material was originally 
arranged bilaterally and ventrally on each side of the central tube, 
exactly in the same way as the nerve-masses of the infra-cesophageal 
and ventral chain of ganglia are arranged with respect to the cephalic 
stomach and straight intestine of the arthropod. But, in addition, we 
find in the vertebrate nervous masses, the cerebral hemispheres, the 
corpora quadrigemina and the cerebellum situated on the dorsal side 
of the central tube in the brain-region ; this nervous material is, 
however, of a different character to that which gives origin to the 
spinal and cranial segmental nerves. How is the presence of these 
dorsal masses to be explained on the supposition that the dilated 
anterior part of the nerve-tube was originally the cephalic stomach 
of the arthropod ancestor? The cerebral hemispheres are simple 
enough, for they represent the supra-cesophageal ganglia, which of 
necessity, as they increased in size, would grow round the anterior 
end of the oephalic stomach and become more and more dorsal in 
position. 

The difficulty lies rather in the position of the cerebellum and 
corpora quadrigemina, and the solution is as simple as it is 
conclusive. 

Let us again turn to embryology and see what help it gives. In 
all vertebrates the dilated anterior portion of the nerve-tube does not, 



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48 THE ORIGIN OF VERTEBRATES • 

as it grows, increase in size uniformly, but a constriction appears on 
its dorsal surface at one particular place, so as to divide it into an 
anterior and posterior vesicle ; then the latter becomes divided into 
two portions by a second constriction. In this way three cerebral 
vesicles are formed ; these three primary cerebral vesicles indicate 
the region of the fore-brain, mid-brain, and hind-brain respectively. 
Subsequently the first cerebral vesicle becomes divided into two to 
form the prosencephalon and thalamencephalon, while the third 
cerebral vesicle is also divided into two to form the region of the 
cerebellum and medulla oblongata. 

These constrictions are in the position of commissural bands of 
nervous matter ; of these the limiting nervous strands between the 
thalamencephalon and mesencephalon and between the mesencephalon 
and the hind-brain are of primary importance. The first of these 
commissural bands is in the position of the posterior commissure 
connecting the two optic thalami. In close connection with this are 
found, on the mid-dorsal region, the two pineal eyes with their optic 
ganglia, the so-called ganglia habenula. From these ganglia a 
peculiar tract of fibre, known as Meynert's bundle, passes on each 
side to the ventral infra-infundibular portion of the brain. In other 
words, the first constriction of the dilated tube is due to the presence 
and growth of nervous material in connection with the median pineal 
eyes. Here in precisely the same spot, as will be fully explained 
in the next chapter, there existed in the arthropod ancestor a pair 
of median eyes situated dorsally to the cephalic stomach, the pre- 
existence of which explains the reason for the first constriction. 

The second primary constriction separating the mid-brain from 
the hind-brain is still more interesting, for it is coincident with the 
position of the trochlear or fourth cranial nerve. In all vertebrates 
without exception this nerve takes an extraordinary course ; all other 
nerves, whether cranial or spinal, pass ventralwards to reach their 
destination. This nerve passes dorsalwards, crosses its fellow mid- 
dorsally in the valve of Vieussens, where the roof of the brain is 
thin, and then passes out to supply the superior oblique muscle of the 
eye of the opposite side. The two nerves form an arch constricting 
the dilated tube at this place. In the lowest vertebrate ( Ammocoetes) 
the constriction formed by this nerve-pair is evident not only in the 
embryonic condition as in other vertebrates, but during the whole 
larval stage. As Fig. 20, A and B, shows, the whole of the dorsal 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 49 

region of the brain up to the region of the pineal eye and ganglion 
habenulce is one large membranous bag, except for the single con- 
striction where the fourth nerve on each side crosses over. The 
explanation of this peculiarity is given in Chapter VII., and follows 
simply from the facts of the arrangement of that musculature in the 
scorpion-group which gave rise to the eye-muscles of the vertebrate. 

In Ammoccetes both cerebellum and posterior corpora quad- 
rigemina can hardly be said to exist, but upon transformation a 
growth of nervous material takes place in this region, and it is seen 
that this commencing cerebellum and the corpora quadrigemina arise 
from tissue that is present in Ammocoetes along the course of the 
fourth nerve. 

Here, then, again Embryology does its best to tell us how the 
vertebrate arose. The formation of the two primary constrictions 
in the dilated anterior vesicle whereby the brain is divided into 
fore-brain, mid-brain, and hind-brain is simply the representation 
ontogenetically of the two nerve-tracts which crossed over the 
cephalic stomach in the prevertebrate stage, in consequence of 
the mid-dorsal position of the pineal eyes and of the insertion of 
the original superior oblique muscles. 

The subsequent constriction by which the prosencephalon is 
separated from the thalamencephalon is in the position of the 
anterior commissure, that commissure which connects the two supra- 
infundibular nerve-masses, and is one of the first-formed commis- 
sures in every vertebrate. This naturally is simply the commissure 
between the two supra-cesophageal ganglia; anterior to it, in the 
middle line, equally naturally, the anterior end of the old stomach 
wall still exists as the lamina terminalis. 

The other division in the hind-brain region, which separates the 
region of the cerebellum from the medulla oblongata, is due to the 
growth of the cerebellum, and indicates its posterior limit. In such 
an animal as the lamprey, where the cerebellum is only commencing, 
this constriction does not occur in the embryo. 

From such simple beginnings as are seen in Ammoccetes, the 
higher forms of brain have been evolved, to culminate in that of man, 
in which the massive cerebrum and cerebellum conceals all sign of 
the dorsal membranous roof, those parts of the simple epithelial tube 
which still remain being tucked away into the cavities to form the 
various choroid plexuses. 



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5° 



THE ORIGIN OF VERTEBRATES 




In the whole evolution from the brain of Ammocoetes to that of 
man, the same process is plainly visible, viz. growth and extension 
of nervous material over the epithelial tube ; extension dorsally and 
posteriorly of the supra-infundibular nervous masses (as seen in 
Fig. 19), combined witli a dorsal growth of parts of the infra- 
infundibular nervous masses to form the cerebellum and posterior 
corpora quadrigemina. 

Especially instructive is the formation of the cerebellum. It 
consists at first of a small mass of nervous tissue accompanying the 
fourth nerve, then by the growth of that mass 
surrounding and constricting a fold of the 
membranous roof, the worm of the cerebellum 
is formed, as in the dog-fish. This very con- 
striction causes the membrane to be thrown 
into a lateral fold on each side, as seen in 
Fig. 24, and in the dog-fish the nervous material 
on each side, known as the fimbria, is already 
commencing to grow from the ventral mass of 
the medulla oblongata to surround these lateral 
membranous folds. These fimbriae develop more 
and more in higher forms, and thus form the 
cerebellar hemispheres. 

Not only does comparative anatomy confirm 
the teachings of embryology, but also pathology 
gives its quota in the same direction. 

One of the striking facts about malforma- 
tions and disease of the central nervous system 
is the frequency of cystic formations; spina 
bifida is a well-known instance. These cysts are merely epithelial 
non-nervous cysts formed from the epithelium of the central canal, 
difficult to understand if the whole nerve tube is one and entirely 
nervous, either actually or potentially, but natural and easy if we 
are really dealing with a simple epithelial tube on the outside of 
which the nervous material was originally grouped. The cystic 
formation belongs naturally enough to this tube, not to the nervous 
system. 

Again, where animals such as lizards have grown a new tail, 
owing to the breaking off of the original one, it is found that the 
central canal extends into this new tail for some distance, but not 



Fig. 24. — Cerebel- 
lum op Dog-fish. 

v, worm of cerebel- 
lum; IV. y membra- 
nous roof of fourth 
ventricle continuous 
with the membra- 
nous folds on each 
side. Through these 
the fimbriae (fb.) can 
be dimly seen. 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 5 I 

the nervous material surrounding it ; all the nerves supplying the 
new tail arise from the uninjured spinal cord above, the central 
canal with its lining layer of epithelial cells alone grows into the 
new-formed appendage. 

To all intents and purposes the same thing is seen in thd termi- 
nation of the spinal cord in a bird-embryo ; more and more, as the 
end of the tail is approached, does the nervous matter of the spinal 
cord grow less and less, until at last a naked central canal with 
its lining epithelium is alone left to represent the so-called nerve- 
tube. 

All these different methods of investigation lead irresistibly to 
the one conclusion that the tubular nature of the central nervous 
system has been caused by the central nervous system enclosing to a 
greater or less extent a pre-existing, non-nervous, epithelial tube. 

This must always be borne strictly in mind. The problem, there- 
fore, which presents itself is the comparison of these two factors 
separately, in order to find out the relationship of the vertebrate to 
the invertebrate. The nervous system without the tube must be 
compared to other nervous systems, and the tube must be considered 
apart from the nervous system. 

The Principle of Concentration and Cephalization. 

The central nervous system of the vertebrate resembles that of 
all the Appendiculata in the fact that it is composed of segments 
joined together which give origin to segmental nerves. There is, 
however, a great difference between the two systems : the division 
into separate segments is not obvious to the eye in the vertebrate 
nervous system, while in the invertebrate we can see that it is 
composed of a series of separate pairs of ganglia joined together 
longitudinally by nervous strands known as connectives and trans- 
versely by the nerve-commissures. Such a simple segmented system 
is found in the segmented worms, and in the lower arthropods, such 
as Branchipus, no great advance has been made on that of the annelid. 
In the higher forms, however, a greater and greater tendency to fusion 
of separate ganglia exists, especially in the head-region, so that the 
infra-cesophageal ganglia, which, in the lower forms are as separate 
as those of the ventral chain, in the higher forms are fused together 
to form a single nervous mass. 



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52 THE ORIGIN OF VERTEBRATES 

This is the great characteristic of the advancement of the central 
nervous system among the Invertebrata, its concentration in the 
region of the head. It may be called the principle of cephalization, 
and is characteristic not only of higher organization in a group, but 
also of the adult as distinguished from the larval form. Thus in the 
imago greater concentration is found than in the caterpillar. 

The segmented annelid type of nervous system consists of a 
supra-cesophageal ganglion, composed of the fused ganglia belonging 
to the pre-oral segments, and an infra-oesophageal chain of separate 
ganglia. With the concentration and modification around the 
mouth of the most anterior locomotor appendages to form organs 
for prehension and mastication of food, a corresponding concentra- 
tion and fusion of the ganglia belonging to these segments takes 
place, so that finally, in the higher annelids, and in most of the great 
arthropod group, a fusion of a number of the most anterior ganglia 
has taken place to form the infra-oesophageal ganglion-mass. 

The infra-ce3ophageal ganglia which are the first to fuse are 
those which supply the most anterior portion of the animal with 
nerves, and include always those anterior appendages which are 
modified for mastication purposes. To this part the name prosoma 
has been given; in many cases it forms a well-defined, distinct 
portion of the animal. 

Succeeding this prosoma or masticatory region, there occurs in 
all gill-bearing arthropods a respiratory region, in many cases more 
or less distinctly defined, which has received the name of mcsosoma. 
The rest of the body is called the metasoma. 

In accordance with this nomenclature the central nervous system 
of many of the Arthropoda may be divided as follows : — 

1. Pre-oral, or supra-oesophageal ganglia. 

2. Infra-oral, or infra-oesophageal ganglia and ventral chain, 
which consist of three groups : prosomatic, mesosomatic, and nieta- 
somatic ganglia. 

The infra-oesophageal ganglion- mass, then, in most of the Arthro- 
poda may be spoken of as formed by the fusion of the prosomatic or 
mouth-ganglia, the mesosomatic and metasomatic remaining separate 
and distinct. The number of ganglia which have fused may be 
observed by examination of the embryo, in which it is easy to see 
indications of the individual ganglia or ncuromcres, although all 
such indication has disappeared in the adult; thus the infra-ceso- 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 53 

phageal ganglia of the cray-fish have been shown to be constituted 
of six prosomatic ganglia. 

In Fig. 25 I give figures of the central nervous system (with the 
exception of the abdominal or metasomatic ganglia) of Branchipus, 
Astacus, Limulus, Scorpio, Androctonus, Thelyphonus, and Ammo- 
coetes. In all the figures the supra-oesophageal ganglia are lined 
horizontally, and their nerves shown, viz. optic (lateral eyes (II) and 
median eyes (II')), olfactory (I) (first antenna?, camerostome, nose) ; 
then come the prosomatic ganglia (dotted), with their nerves (A) 
supplying the mouth parts, and the second antennae or chelicerae ; 
then the mesosomatic (lined horizontally), with their nerves (B) 
supplying respiratory appendages. These figures show that the con- 
centrated brain mass around the oesophagus of an arthropod which 
has arrived at the stage of Astacus, is represented by the supra- 
oesophageal ganglia and the fused prosomatic ganglia. 

The next stage in the evolution of the brain is seen in the 
gradual in lusion of the mesosomatic ganglia, one after the other, 
into the infra-oesophageal mass of the already fused prosomatic 
ganglia. With this fusion is associated the loss of locomotion in 
these mesosomatic appendages, and their entire subservience to the 
function of respiration. Dana urges that cephalization is a conse- 
quence of functional alteration in the appendages, from organs of 
locomotion to those of mastication and respiration. Whether this be 
true or not, it is certainly a fact that in Limulus, the ganglion 
supplying the first mesosomatic appendage has fused with the 
prosomatic, infra-oesophageal mass. It is also a fact that the proso- 
matic appendages are the organs of mastication, their basal parts 
being arranged round the mouth so as to act as foot-jaws, while the 
mesosomatic appendages, though still free to move, have been 
reduced to such an extent as to consist mainly of their basal parts, 
which are all respiratory in function, except in the case of the first 
pair, where they carry the terminal ducts of the genital organs. In 
the next stage, that, of the scorpion, in which the mesosomatic 
appendages have lost all power of free locomotion, and have become 
internal branchiae, another mesosomatic ganglion has fused with the 
brain mass, while in Androctonus two of the branchial mesosomatic 
ganglia have fused ; and finally, in Thelyphonus and Phrynus, all 
the mesosomatic ganglia have coalesced with the fused prosomatic 
ganglia, while the metasomatic ganglia have themselves fused 



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54 



THE ORIGIN OF VERTEBRATES 




ANDROCTONUS 



AMMOCCETES 

Fig. 25. — Comparison of Invertebrate Brains from Branchipus to 

Ammoccetes. 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 55 

together in the caudal region to form what is known as the caudal 
brain. 

The brain in these animals may be spoken of as composed of 
three parts — (1) the fused supra-cesophageal ganglia, (2) the fused 
prosomatic ganglia, and (3) the fused mesosomatic ganglia. Such a 
brain is strictly homologous with the vertebrate brain, which also is 
built up of three parts— (1) the part in front of the notochord, the 
prechordal or supra-infundibular brain, which consists of the. cerebral 
hemispheres, together with the basal and optic ganglia and corre- 
sponds, therefore, to the supra-cesophageal mass, with its olfactory 
and optic divisions lying in front of the oesophagus ; (2 and 3) the 
epichordal brain, composed of (2) a trigeminal and (3) a vagus divi- 
sion, of which the first corresponds strictly to the fused prosomatic 
ganglia, and the second to the fused mesosomatic ganglia. Further, 
just as in the embryo of an arthropod it is possible, with more or 
less accuracy, to see the number of neuromeres or original ganglia 
which have fused to form the supra- and infra- oesophageal portions 
of its brain, so also in the embryo of a vertebrate we are able at 
an early stage to gain an indication, more or less accurate, of the 
number of neuromeres which have built up the vertebrate brain. 
The further consideration of these neuromeres, and the evidence they 
afford as to the number of the prosomatic and mesosomatic ganglia 
which have formed the epichordal part of the vertebrate brain, must 
be left to the chapter on the segmentation of the cranial nerves. 

The further continuation of this process of concentration of 
separate segments, together with the fusion of the nervous system 
with the tube of the alimentary canal, leads in the simplest manner 
to the formation of the spinal cord of the vertebrate from the meta- 
somatic ganglia of the ventral chain of the arthropod. 

The Antagonism between Cephalization and Alimentation. 

This concentration of the nervous system in the head-region, 
together with an actual increase in the bulk of the cephalic nervous 
masses, constitutes the great principle upon which the law of upward 
progress or evolution in the animal kingdom is based, and it illus- 
trates in a striking manner the blind way in which natural selection 
works; for, as already explained, the central nervous system arose as 
a ring round the mouth, in consequence of which, with the progressive 



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56 



THE ORIGIN OF VERTEBRATES 



evolution of the animal kingdom, the oesophagus necessarily pierced 
the central nervous system at the cephalic end. At the same time, 
the very fact that the evolution was progressive necessitated the 
concentration and increase of the nervous masses in this very same 
oesophageal region. 

Progress on these lines must result in a crisis, owing to the 
inevitable squeezing out of the food-channel by the increasing nerve- 
mass ; and, indeed, the fact that such a crisis h&d in all probability 
arisen at the time when vertebrates first appeared is apparent when 
we examine the conditions at the present time. 

Those invertebrates whose central nervous system is most con- 
centrated at the cephalic end belong to the arachnid group, among 
which are included the various living scorpion-like animals, such as 
Thelyphonus, Androctonus, etc. 

As already mentioned, the giants of the Palseostracan age were 
Pterygotus, Slimonia, etc., all animals of the scorpion-type — in fact, 
A sea - scorpions. Now, all these 

animals, spiders and scorpions, 
without exception, are blood - 
suckers, and in all of them the 
concentrated cephalic mass of ner- 
vous material surrounds an oeso- 
phagus the calibre of which is so 
small that nothing but a fluid 
pabulum can be taken into the 
alimentary canal; and even for 
that purpose a special suctorial 
apparatus has in some species 
been formed on the gastric side 
of the oesophagus for the purpose 
of drawing blood through thi3 
exceedingly narrow tube. 

In Fig. 25 this increasing 
antagonism between brain-power 
and alimentation, as we pass from 
such a form as Branchipus to the 
scorpion, is illustrated, and in Fig. 2G the relative sizes of the 
oesophagus and the brain-mass surrounding it is shown. The section 
shows that the food channel is surrounded by the white and grey 




mi*.,: 







B 

Fio. 26. -Transvkrs e Section 
titrough the brain of a young 
Thelyphonus. 

^4, supra-oesophageal ganglia; 7?, infra- 
opsophagcal ganglia; Al y oesophagus. 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 57 

matter of the brain as completely as the central canal of the spinal 
cord of the vertebrate is surrounded by the white and grey nervous 
material. 

Truly, at the time when vertebrates first appeared, the direction 
and progress of variation in the Arthropoda was leading, owing to 
the manner in which the brain was pierced by the oesophagus, to a 
terrible dilemma — either the capacity for taking in food without 
sufficient intelligence to capture it, or intelligence sufficient to capture 
food and no power to consume it. 

Something had to be done — some way had to be found out of this 
difficulty. The atrophy of the brain meant degeneration and the 
reduction to a lower stage of organization, as is seen in the Tunicata. 
The further development of the brain necessitated the establish- 
ment of a new method of alimentation and the closure of the old 
cesophagus, its vestiges still remaining as the infundibular canal of 
the vertebrate, meant the enormous upward stride of the formation 
of the vertebrate. 

At first sight it might appear too great an assumption even to 
imagine the possibility of the formation of a new gut in an animal so 
highly organized as an arthropod, but a little consideration will, I 
think, show that such is not the case. 

In the higher animals we are accustomed to speak of certain 
organs as vital and necessary for the further existence of the animal ; 
these are essentially the central nervous system, the respiratory 
system, the circulatory system, and the digestive system. Of these 
four vital systems the first cannot be touched without the chance 
of degeneration ; but that is not the case with the second. The 
passage from the fish to the amphibian, from the water-breathing 
to the air-breathing animal, has actually taken place, and was effected 
by the modification of the swim-bladder to form new respiratory 
organs — the lungs ; the old respiratory organs — the gills — becoming 
functionless, but still persisting in the embryo as vestiges. The 
necessity arose in consequence of the passage of the animal from 
water to land, and with this necessity nature found a means of over- 
coming the difficulty ; air-breathing vertebrates arose, and from the 
very fact of their being able to extend over the land-surfaces, 
increased in numbers and developed in complexity in the manner 
already sketched out. 

For a respiratory system all that is required is an arrangement 



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58 THE ORIGIN OF VERTEBRATES 

by means of which blood should be brought to the surface, so as to 
interchange its gases with those of the external medium ; and it is 
significant to find that of all vertebrates the Amphibia alone are 
capable of an effective respiration by means of the skin. 

As to the circulatory system, it is exceedingly easily modified. 
An animal such as Amphioxus has no heart ; in some the heart is 
systemic, in others branchial ; in some there are more than one heart ; 
in others there are contractile veins in addition to a heart. There 
is no difficulty here in altering and modifying the system according 
to the needs of the individual. 

For a digestive system all that is required is an arrangement for 
the digestion and absorption of food, a mechanism which can arise 
easily if some of the cells of the skin possess digestive power. Now 
Miss Alcock has shown that some of the surface-cells of crustaceans 
secrete a fluid which possesses digestive powers, and she has also 
shown that certain of the cells in the skin of Ammoccetes possess 
digestive power. 

The difficulty, then, of forming a new digestive system in the 
passage from the arthropod to the vertebrate is very much the same 
as the difficulty in forming a new respiratory system in the passage 
from the water-breathing fish to the air-breathing amphibian — a 
change which does not strike us as inconceivable, because we know it 
has taken place. 

The whole argument so far leads to the conclusion that vertebrates 
arose from ancient forms of arthropods by the formation of a new 
alimentary canal, and the enclosure of the old canal by the growing 
central nervous system. If this conclusion is true, then it follows 
that we possess a well-defined starting-point from which to compare 
the separate organs of the arthropod with those of the vertebrate, 
and if, in consequence of such working hypothesis, each organ of the 
arthropod is found in the vertebrate in a corresponding position and 
of similar structure, then the truth of the starting-point is proved as 
fully as can possibly be expected by deductive methods. It is, in 
fact, this method of comparative anatomy which has proved the 
descent of man from the ape, the frog from the fish, etc. 

Let us, then, compare all the organs of such a low vertebrate as 
Ammoccetes with those of an arthropod of the ancient type. 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 59 

Life History of the Lamprey— not a Degenerate Animal. 

The striking peculiarity of the lamprey is its life-history. It 
lives in fresh water, spending a large portion of its life in the mud 
during the period of its larval existence : then comes a somewhat 
sudden transformation-stage, characterized, as in the lepidopterous 
larva, by a process of histolysis, by which many of the larval tissues 
are destroyed and new ones formed, with the result that the larval 
lamprey, or Ammoccetes, is transformed into the adult lamprey, or 
Petromyzon. This transformation takes place in August, at all 
events in the neighbourhood of Cambridge, and later in the year the 
transformed lamprey migrates to the sea, grows in size and maturity, 
and returns to the river the following spring up to its spawning beds, 
where it spawns and forthwith dies. How long it lives in the Ammo- 
ccetes stage is unknown ; I myself have kept some without transfor- 
mation for four years, and probably they live in the rivers longer 
than that before they change from their larval state. It is absolutely 
certain that very much the longest part of the animal's life is spent 
in the larval stage, and that with the maturity of the sexual organs 
and the production of the fertilized ova the life of the individual ends. 

Now, the striking point of this transformation is that it produces 
an animal more nearly comparable with higher vertebrates than is 
the larval form ; in other words, the transformation from larva to 
adult is in the direction of upward progress, not of degeneration. 
It is, therefore, inaccurate to speak of the adult lamprey as 
degenerate from a higher race of fishes represented by its larval form 
— Ammoccetes. Its transformation does not resemble that of the 
tunicates, but rather that of the frog, so that, just as in the case of 
the tadpole, the peculiarities of its larval form may be expected to 
afford valuable indications of its immediate ancestry. The very 
peculiarities to which attention must especially be paid are those 
discarded at transformation, and, as will be seen, these are essentially 
characteristic of the invertebrate and are not found in the higher 
vertebrates. In fact, the transformation of the lamprey from the 
Ammoccetes to the Petromyzon stage may be described as the casting 
off of many of its ancestral invertebrate characters and the putting 
on of the characteristics of the vertebrate type. It is this double 
individuality of the lamprey, together with its long-continued 
existence in the larval form, which makes Ammocoetes more 



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60 THE ORIGIN OF VERTEBRATES 

valuable than any other living vertebrate for the study of the stock 
from which vertebrates sprang. 

Many authorities hold the view that the lamprey, like Amphioxus, 
must be looked upon as degenerate, and therefore as no more suitable 
for the investigation of the problem of vertebrate ancestry than is 
Amphioxus itself. This charge of degeneracy is based on the state- 
ment that the lamprey is a parasite, and that the eyes in Ammocoetes 
are under the skin. The whole supposition of the degeneracy of the 
Cyclostomata arose because of the prevailing belief of the time that 
the earliest fishes were elasmobranchs, and therefore gnathosto- 
matous. From such gnathostomatous fishes the cyclostomes were 
supposed to have descended, having lost their jaws and become 
suctorial in habit in consequence of their parasitism. 

The charge of parasitism is brought against the lamprey because 
it is said to suck on to fishes and so obtain nutriment. It is, how- 
ever, undoubtedly a free-swimming fish ; and when we see it coming 
up the rivers in thousands to reach the spawning-beds, and sucking 
on to the stones on the way in order to anchor itself against the 
current, or holding on tightly during the actual process of spawning, 
it does not seem justifiable to base a charge of degeneration upon a 
parasitic habit, when such so-called habit simply consists in holding 
on to its prey until its desires are satisfied. If, of course, its suctorial 
mouth had arisen from an ancestral gnathostomatous mouth, then 
the argument would have more force. 

Dohrn, however, gives absolutely no evidence of a former 
gnathostomotous condition either in Petromyzon or, in its larval 
state, Ammoccetes. He simply assumes that the Cyclostomata are 
degenerated fishes and then proceeds to point out the rudiments of 
skeleton, etc., which they still possess. Every point that Dohrn 
makes can be turned round ; and, with more probability, it can be 
argued that the various structures are the commencement of the 
skeletal and other structures in the higher fishes, and not their 
degenerated remnants. Compare the life-history of the lamprey 
and of the tunicate. In the latter case we look upon the animal as 
a degenerate vertebrate, because the larval stage alone shows verte- 
brate characteristics ; when transformation has taken place, and the 
adult form is reached, the vertebrate characteristics have vanished, 
nnd the animal, instead of reaching a higher grade, has sunk lower 
in the scale, the central nervous system especially having lost all 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 6 1 

resemblance to that of the vertebrate. In the former case a trans- 
formation also takes place, a marvellous transformation, characterized 
by two most striking facts. On the one hand, the resulting animal 
is more like a higher vertebrate, for, by the formation of new 
cartilages, its cranial skeleton is now comparable with that of the 
higher forms, and the beginnings of the spinal vertebra) appear ; by 
the increased formation of nervous material, its brain increases in 
size and complexity, so as to compare more closely with higher 
vertebrate brains ; its eyes become functional, and its branchiae are 
so modified, simultaneously with the formation of the new alimentary 
canal in the cranial region, that they now surround branchial pouches 
which are directly comparable to those of higher vertebrates. On 
the other hand, the transformation process is equally characterized 
by the throwing off of tissues and organs, one and all of which are 
comparable in structure and function with corresponding structures in 
the Arthropoda— the thyroid of the Ammoccetes, the tentacles, the 
muco-cartilage, the tubular muscles, all these structures, so striking 
in the Ammoccetes stage, are got rid of at transformation. Here is 
the true clue. Here, in the throwing off of invertebrate characters, 
and the taking on of a higher vertebrate form, especially a higher 
brain, not a lower one, Petromyzon proclaims as clearly as is possible 
that it is not a degenerate elasmobranch, but that it has arisen from 
Ammoccetes-like ancestors, even though Myxine, Amphioxus, and 
the tunicates be all stages on the downward grade from those same 
AmmocoBtes-like ancestors. 

As to the eyes, they are functional in the adult form and as service- 
able as in any fish. There is no sign of degeneracy; it is only possible 
to speak of a retarded development which lasts through the larval stage. 

Comparison of Brain of Ammoccetes with that of an 
Arthropod. 

Seeing that the steady progress of the development of the central 
nervous system is the most important factor in the evolution of 
animals, it follows that of all organs of the body, the central nervous 
system must be most easily comparable with that of the supposed 
ancestor. I will, therefore, start by comparing the brain of 
Ammoccetes with that of arthropods, especially of Limulus aud of 
the scorpion-group. 



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62 



THE ORIGIN OF VERTEBRATES 



The supra-infundibular portion of the brain in vertebrates 
corresponds clearly to the supra-cesophageal portion of the inverte- 
brate brain in so far that in both cases here is the seat of the will. 
Voluntary action is as impossible to the arthropod deprived of its 
supra-oesophageal ganglia as to the vertebrate deprived of its cere- 
brum. It corresponds, also, in that from it arise the nerves of sight 
and smell and no other nerves ; this is also the case with the supra- 
cesophageal ganglia, for from a portion of these ganglia arise the nerves 
to the eyes and the nerves to the first antennae, of which the latter 
are olfactory in function. Thus, in the accompanying figure, taken 
from Bellonci, it is seen that the supra-cesophageal ganglia consist 



Sup. Segment Ant I 




Fig. 27. — The Brain of SpJueroma serratum. (After Bellonci.) 

Ant. I. and Ant. II., nerves to 1st and 2nd antenna, f.br.r., terminal fibre layer of 
retina; Op. g. /., first optic ganglion; Op. g. II. , second optic ganglion; O.n., 
optic nerve-fibres forming an optic chiasma. 

of a superior segment corresponding to the cerebrum, a middle 
segment from which arise the nerves to the lateral eyes and to the 
olfactory antenme, corresponding to the basal ganglia of the brain 
and the optic lobes, and, according to Bellonci, of an inferior segment 
from which arise the nerves to the second pair of antennas. This 
last segment is not supra-cesophageal in position, but is situated on 
the oesophageal commissures. It has been shown by Lankester and 
Brauer in Limulus and the scorpion to be in reality the first ganglion 
of the infra-oesophageal scries, and not to belong to the supra- 
cesophageal group. 

Further, in Limulus, in the scorpion-group, and in all the extinct 



v 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 63 

Eurypteridie— in fact, in the Palaeostraca generally — there are two 
median eyes in addition to the lateral eyes, which were innervated 
from these ganglia. 

In Ammoccetes, then, if the supra-infundibular portion of the 
brain really corresponds to the supra-oesophageal of the palaeostracan 
group, we ought to find, as indeed is the case, an optic apparatus 
consisting of two lateral eyes and two median eyes, innervated from 
the supra-infundibular brain-mass, and an olfactory apparatus built 
up on the same lines as in the scorpion-group, also innervated from 
this region. If, in addition, it be found that those two median eyes 
are degenerate eyes of the same type as the median eyes of Limulus 
and the scorpion-group, then the evidence is so strong as to amount 
to a proof of the correctness of the theory. This evidence is precisely 
what has been obtained in recent years, for the vertebrate did possess 
two median eyes in addition to the two lateral ones, and these two 
median eyes are degenerate eyes of the type found in the median 
eyes of arthropods and are not of the vertebrate type. Moreover, as 
ought also to be the case, they are most evident, and one of the 
pair is most nearly functional in the lowest perfect vertebrate, 
Ammoccetes. 

Of all the discoveries made in recent years, the discovery that 
the pineal gland of the vertebrate brain was originally a pair of 
median eyes is by far the most important clue to the ancestry of 
the vertebrate, for not only do they correspond exactly in position 
with the median eyes of the invertebrates, but, being already 
degenerate and function less in the lowest vertebrate, they must have 
been functional in a pre-vertebrate stage, thus giving the most direct 
clue possible to the nature of the pre-vertebrate stage. It is 
especially significant that in Limulus they are already partially 
degenerated. What, then, ought to be the structure and relation to 
the brain of the median and lateral eyes of the vertebrate if they 
originated from the corresponding organs of some one or other member 
of the palaeostracan group ? 

This question will form the subject of the next chapter. 

Summaky. 

The object of this book is to attempt to find out from what tfroup of inverte- 
brates the vertebrate arose ; no attempt is made to speculate upon the causes of 
variation by means of which evolution takes place. 



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64 THE ORIGIN OF VERTEBRATES 

A review of the animal kingdom as a whole leads to the conclusion that the 
upward development of animals from an original coelenterate stock, in which 
the central nervous system consists of a ring of nervous material surrounding 
the mouth, has led, in consequence of the elaboration of the central nervous 
system, to a general plan among the higher groups of invertebrates in the topo- 
graphical arrangement of the important organs. The mouth is situated ventrally, 
and leads by means of the oesophagus into an alimentary canal which is situated 
dorsally to the central nervous system. Thus the oesophagus pierces the central 
nervous system and divides it into two parts, the supra-oesophageal ganglia 
and the infra-oesophageal ganglia. This is an almost universal plan among 
invertebrates, but apparently does not hold for vertebrates, for in them the 
central nervous system is always situated dorsally and the alimentary canal 
ventrally, and there is no piercing of the central nervous system by an oesophagus. 

Yet a remarkable resemblance exists between the central nervous system of 
the vertebrate and that of the higher invertebrates, of so striking a character as 
to compel one school of anatomists to attempt the derivation of vertebrates 
from annelids. Now, the central nervous system of vertebrates forms a hollow 
tube, and a diverticulum of this hollow tube, known as the infundibulum, passes 
to the ventral surface of the brain in the very position where the oesophagus 
would have been if that brain had belonged to an annelid or an arthropod. 
This school of anatomists therefore concluded that this infundibular tube 
represented the original invertebrate oesophagus which had become closed and 
no longer opened into the alimentary canal owing to the formation of a new 
mouth in the vertebrate. As, however, the alimentary canal of the vertebrate 
is ventral to the central nervous system, and not dorsal, as in the invertebrate, 
it follows that the remains of the original invertebrate mouth into which the 
oesophagus (in the vertebrate the infundibular tube) must have opened must be 
searched for on the dorsal side of the vertebrate ; and so the theory was put 
forward that the vertebrate had arisen from the annelid by the reversal of 
surfaces, the back of the one animal becoming the front of the other. 

The difficulties in the way of accepting such reversal of surfaces have proved 
insuperable, and another school has arisen which suggests that evolution has 
throughout proceeded on two lines, the one forming groups of animals in which 
the central nervous system is pierced by the food-channel and the gut therefore 
lies dorsally to it, the other in which the central nervous system always lies 
dorsally to the alimentary canal and is not pierced by it. In both cases the 
highest products of the evolution have become markedly segmented animals, in 
the former, annelids and arthropods ; in the latter, vertebrates. The only 
evidence on which such theory is based is the existence of low forms of animals, 
known as the Enteropneusta, the best known example of which is called 
Balanoglossus ; they are looked upon as aberrant annelid forms by many 
observers. 

This theory does not attempt to explain the peculiarities of the tube of the 
vertebrate central nervous system, or to account for the extraordinary resemblance 
between the structure and arrangement of the central nervous systems of 
vertebrates and of the highest invertebrate group. 

Neither of these theories is satisfactory or has secured universal acceptance. 
The problem must be considered entirely anew. What are the guiding principles 
in this investigation H 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 65 

The evolution of animal life on this earth can clearly, on the whole, be 
described as a process of upward progress culminating in the highest form — 
man ; but it must always be remembered that whole groups of animals such as the 
Tunicata have been able to survive owing to a reverse process of degeneration. 

If there is one organ more than another which increases in complexity as 
evolution proceeds, which is the most essential organ for upward progress, surely 
it is the central nervous system, especially that portion of it called the brain. 
This consideration points directly to the origin of vertebrates from the most 
highly organized invertebrate group — the Arthropoda — for among all the groups 
of animals living on the earth in the present day they alone possess a central 
nervous system closely comparable with that of vertebrates. Not only has an 
upward progress taken place in animals as a whole, but also in the tissues them- 
selves a similar evolution is apparent, and the evidence shows that the vertebrate 
tissues resemble more closely those of the arthropod than of any other inverte- 
brate group. 

The evidence of geology points to the same conclusion, for the evidence of 
the rocks shows that before the highest mammal — man — appeared, the dominant 
race was the mammalian quadruped, from whom the highest mammal of all — 
man— sprung; then comes, in Mesozoic times, the age of reptiles which were 
dominant when the mammal arose from them. Preceding this era we find in 
Carboniferous times that the amphibian was dominant, and from them the next 
higher group— the reptiles— arose. Below the Carboniferous come the Devonian 
strata with their evidence of the dominance of the fish, from whom the 
amphibian was directly evolved. The evidence is so clear that each succeeding 
higher form of vertebrate arose from the highest stage reached at the time, 
as to compel one to the conclusion that the fishes arose from the race which 
was dominant at the time when the fishes first appeared. This brings us to the 
Silurian age, in which the evidence of the rocks points unmistakably to the sea- 
scorpions, king-crabs, and trilobites as being the dominant race. It was preceded 
by the great trilobite age, and the whole period, from the first appearance of the 
trilobite to the time of dwindling away of the sea-scorpions, may be designated 
the Palseostracan age, using the term Palaeostraca to include both trilobites and 
the higher scorpion and king-crab forms evolved from them. The evidence of 
geology then points directly and strongly to the origin of vertebrates from the 
Palseostraca — arthropod forms which were not crustacean and not arachnid, 
but gave origin both to the modern-day crustaceans and arachnids. The 
history of the rocks further shows that these ancient fishes, when they first 
appeared, resembled in a remarkable manner members of the palceostracan group, 
so that again and again palaeontologists have found great difficulty in determin- 
ing whether a fossil is a fish or an arthropod. Fortunately, there is still alive 
on the earth one member of this remarkable group— the Lunulas, or King- 
Crab. On the vertebrate side the lowest non-degenerate vertebrate is the 
lamprey, or Petromyzon, which spends a large portion of its existence in a 
larval stage, known as the Ammocoetes stage of the lamprey, because it was 
formerly considered to be a separate species and received the name of 
Ammocoetes. The larval stages of any animal are most valuable for the study 
of ancestral problems, so that it is most fortunate for the solution of the ancestry 
of vertebrates that Hamulus on the one side and Ammocoetes on the other are 

F 



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66 THE ORIGIN OF VERTEBRATES 

available for thorough investigation and comparison. There are no trilobites 
still alive, but in Branchipus and Apus we possess the nearest approach to the 
trilobite organization among living crustaceans. 

So strongly do all these different lines of argument point to the origin of 
vertebrates from arthropods as to make it imperative to reconsider the position 
of that school of anatomists who derived vertebrates from annelids by reversing 
the back and front of the animal. Let us not turn the animal over, but 
re-consider the position, the infundibular tube of the vertebrate still representing 
the oesophagus of the invertebrate, the cerebral hemispheres and basal ganglia 
the supra-oesophageal ganglia, the crura cerebri the oesophageal commissures, 
and the infra-infundibular part of the brain the infra-oesophageal ganglia. It 
is immediately apparent that just as the invertebrate oesophagus leads into the 
large cephalic stomach, so the infundibular tube leads into the large cavity of 
the brain known as the third ventricle, which, together with the other ventricles, 
forms in the embryo a large anterior dilated part of the neural tube. In the 
arthropod this cephalic stomach leads into the straight narrow intestine ; in the 
vertebrate the fourth ventricle leads into the straight narrow canal of the spinal 
cord. In the arthropod the intestine terminates in the anus ; in the vertebrate 
embryo the canal of the spinal cord terminates in the anus by way of the 
neurenteric canal. Keep the animal unreversed, and immediately the whole 
mystery of the tubular nature of the central nervous system is revealed, for it 
is seen that the nervous matter, which corresponds bit by bit with that of the 
arthropod, has surrounded to a greater or less extent and amalgamated with 
the tube of the arthropod alimentary canal, and thus formed the so-called 
central nervous system of the vertebrate. 

The manner in which the nervous material has invaded the walls of the tube 
is clearly shown both by the study of the comparative anatomy of the central 
nervous system in the vertebrate and also by its development in the embryo. 

This theory implies that the vertebrate alimentary canal is a new formation 
necessitated by the urgency of the case, and, indeed, there was cause for urgency, 
for the general plan of the evolution of the invertebrate from the coelenterate 
involved the piercing of the anterior portion of the central nervous system by the 
oesophagus, while, at the same time, upward progress meant brain-development ; 
brain-development meant concentration of nervous matter at the anterior end 
of the animal, with the result that in the highest scorpion and spider-like 
animals the brain-mass has so grown round and compressed the food-tube that 
nothing but fluid pabulum can pass through into the stomach ; the whole group 
have become blood-suckers. These kinds of animal a — the sea-scorpions— were 
the dominant race when the vertebrates first appeared : here in the natural com- 
petition among members of the dominant race the difficulty must have become 
acute. Further upward evolution demanded a larger and larger brain with the 
ensuing consequence of a greater and greater difficulty of food-supply. Nature's 
mistake was rectified and further evolution secured, not by degeneration in the 
brain-region, for that means degradation not upward progress, but by the 
formation of a new food-channel, in consequence of which the brain was free 
to develop to its fullest extent. Thus the great and mighty kingdom of the 
^Vertebrata was evolved with its culminating organism— man —whose massive 
with all its possibilities could never have been evolved if he had still been 



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THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 67 

compelled to pass the whole of his food through the narrow oesophageal tube, 
still existent in him as the infundibular tube. This, then, is the working 
hypothesis upon which this book is written. If this view is right, that the 
Vertebrate was formed from the Palaeostracan without any reversal of surfaces, 
but by the amalgamation of the central nervous system and alimentary canal, 
then it follows that we have various fixed points of comparison in the central 
nervous systems of the two groups of animals from which to search for further 
clues. It further f ollows that from such starting-point every organ of importance 
in the body of the arthropod ought to be visible in the corresponding position in 
the vertebrate, either as a functional or rudimentary organ. The subsequent 
chapters will deal with this detailed comparison of organs in the arthropod and 
vertebrate respectively. 



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I; 



\ 



CHAPTER II 
THE EVIDENCE OF THE ORGANS OF VISION 

Different kinds of eye. — Simple and compound retinas.— Upright and inverted 
retinas. — Median eyes. — Median or pineal eyes of Ammocoetes and their 
optic ganglia.— Comparison with other median eyes. — Lateral eyes of verte- 
brates compared with lateral eyes of crustaceans. — Peculiarities of the 
lateral eye of the lamprey. — Meaning of the optic diverticula. — Evolution 
of vertebrate eyes. — Summary. 

The Different Kinds of Eye. 

In all animals the eyes are composed of two parts. 1. A set of 
special sensory cells called the retina. 2. A dioptric apparatus for 
the purpose of forming an image on the sensory cells. The simplest 
eye is formed from a modified patch of the surface-epithelium ; cer- 
tain of the hypodermal cells, as they are called, elongate, and their 
cuticular surface becomes bulged to form a simple lens. These 
elongated cells form the retinal cells, and are connected with the 
central nervous system by nerve-fibres which constitute an optic 
nerve ; the cells themselves may contain pigment. 

The more complicated eyes are modifications of this type for the 
purpose of making both the retina and the dioptric apparatus more 
perfect. According to a very prevalent view, these modifications have 
been brought about by invaginations of the surface-epithelium. 
Thus if ABCD (Fig. 28) represents a portion of the surface-epithelium, 
the chitinous cuticle being represented by the dark line, with 
the hypodermal cells beneath, and if the part C is modified to form 
an optic sense-plate, then an invagination occurring between A and B 
will throw the retinal sense-cells with the optic nerve further from 
the surface, and the layers B and A between the retina and the source 
of light will be available for the formation of the dioptric apparatus. 
The lens is now formed from the cuticular surface of A, and the 



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THE EVIDENCE OF THE ORGANS OF VISION 



6 9 



hypodermal cells of A elongate to form the layer known by the name 
of corneagen, or vitreogen, the cells of B remaining small and forming 
the pre-retinal layer of cells. The large optic nerve end- cells of the 
retinal layer, C, take up the position shown in the figure, and their 
cuticular surface becomes modified to form rods of varying shape 
called rhabdites, which are attached to the retinal cells. Frequently 
the rhabdites of neighbouring cells form definite groups, each group 
being called a rhabdome. Whatever shape they take it is invariably 
found that these little rods (bacilli), or rhabdites, are modifications of 
the cuticular surface of the cells which form the retinal layer. Also, 
as must necessarily be the case from the method of formation, the 
optic nerve arises from the nuclear end of the retinal cells, never from 

1 





Fig. 28.— Diagram op Formation of an Upright Simple Retina. 

the bacillary end. As in the case first mentioned, so in this case, the 
light strikes direct upon the bacillary end of the retinal cells ; such 
eyes, therefore, are eyes with an upright retina. 

It may happen that the part invaginated is the optic sense-plate 
itself, as would be the case if in the former figure, instead of C, the 
part B was modified to form a sense-plate. This will give rise to 
an eye of a character different from the former (Fig. 29). The optic 
nerve- fibres now lie between the source of light and the retinal end- 
cells, the layer A as before forms the cuticular lens, and its hypo- 
dermal cells elongate to form the corneagen ; there is no pre-retinal 
layer, but, on the contrary, a post-retinal layer, C, called the tapetum, 
and, as is seen, the light passes through the retinal layer to the 



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7o 



THE ORIGIN OF VERTEBRATES 



tapetum. The cuticular surface of the retinal cells forming the rods 
or bacilli is directed towards the tapetal layer away from the source of 
light, and the nuclei of the retinal cells are pre-bacillary in position, 
in contradistinction to the upright eye, where they are post-bacillary. 
The retinal end-cells are devoid of pigment, the pigment being in the 
tapetal layer. 

Such an eye, in contradistinction to the former type, is an eye 
with an inverted retina ; but still the same law holds as in the former 
case — the optic nerve-fibres enter at the nuclear ends of the cells, 
and the rods are formed from the cuticular surface. 

In these eyes the pigmented tapetal layer is believed to act as a 
looking-glass; the dioptric apparatus throws the image on to its 

I 





Fig. 29. — Diagram of Formation of an Inverted Simple Retina. 

The arrow shows the direction of the source of light in this as in the preceding figure. 
In both figures the cuticular rhabdites are represented by thick black lines. 

shiny surface, from whence it is reflected directly on to the rods, 
which are in close contact with the tapetum. A similar process has 
been suggested in the case of the mammalian lateral eye, with its 
inverted retina. Johnson describes the post-retinal pigmented layer 
as being frequently coloured and shiny, and imagines that it reflects 
the image directly back on to the rods. 

Thus we see that eyes can be placed in different categories, e.g. 
those with an upright retina and those with an inverted retina ; also, 
according to the presence or absence of a tapetum, eyes have been 
grouped as tapetal or non-tapetal. All the eyes considered so far 
are called simple eyes, or ocelli; and a number of ocelli may be 



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THE EVIDENCE OF THE ORGANS OF VISION 7 1 

contiguous though separate, as in the lateral eyes of the scorpion. 
They may, however, come into close contact and form one single, 
large, compound eye. Such ocelli, in a very large number of cases, 
retain each its own dioptric apparatus, and therefore the external 
appearance of the compound eye represents not a single lens, but a 
large number of facets, as is seen in the eyes of insects. Owing to 
these differences, eyes have been divided into simple and compound, 
and into facetted and non-facetted. 

Yet another complication occurs in the formation of eyes, which 
is, perhaps, the most important of all : the retinal portion of the eye, 
instead of consisting of simple retinal cells, with their accompanying 
rhabdites, may include within itself a portioA of the central nervous 
system. 

The rationale of such a formation is as follows: The external 
covering of the body is formed by a layer of external epithelial cells 
— the ectodermal cell-layer — and an underlying neural layer, of which 
the latter gives origin to the central nervous system. As development 
proceeds, this central nervous system sinks inwards, leaving as its 
connection with the ectoderm the sensory nerves of the skin. That 
part of the neural layer which underlies the optic plate forms the 
optic ganglion, and when the central nervous system leaves the 
surface to take up its deeper position, the strand of nerve-fibres 
known as the optic nerve, is left connecting it with the retinal cells 
as seen in Figs. 28, 29. It may, however, happen that part of the 
optic ganglion remains at the surface, in close connection with the 
retinal end-cells, when the rest of the central nervous system sinks 
inwards. The retina of such an eye is composed of the combined 
optic ganglion and retinal end-cells ; the strand of nerve-fibres which 
is left as the connection between it and the rest of the brain, which 
is also called the optic nerve, is not a true peripheral nerve, as in 
the first case, but rather a tract of fibres connecting two parts of the 
brain, of which one has remained at the periphery. Such a retina, 
in contradistinction to the first kind, may be called a compound retina. 

The optic ganglion, as seen in eyes with a simple retina, consists 
of a cortical layer of small, round nerve-cells, and an internal medulla 
of fine nerve-fibres, which form a thick network known as 'Punct- 
substanz,' or in modern terminology, 'Neuropil/ Fibres which pass 
into this 'neuropil* from other parts of the brain connect them 
with the optic ganglion. 



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72 THE ORIGIN OF VERTEBRATES 

At the present time, owing to the researches of Golgi, Ramon y 
Cajal, and others, the nervous system is considered to be composed 
of a number of separate nerve-units, called neurones, each neurone 
consisting of a nerve-cell with its various processes; one of these 
— the neuraxon — constitutes the nerve-fibre belonging to that nerve- 
cell, the other processes— the dendrites — establish communication 
with other neurones. The place where these processes come together 
is called a synapse, and the tangle of fine fibres formed at a number 
of synapses forms the ' neuropil/ 

When, therefore, a compound retina is formed by the amalgama- 
tion of the ectodermal part — the retinal cells proper — with the 
neurodermic part — to which the name 'retinal ganglion ' may be 



Fig. 30.— Diagram of Formation of an Upright Compound Retina. 

ABCD, as in Fig. 28. Op. g. L and Op. g. II. , two optic ganglia which combine 
to form the retinal ganglion, Rt. g. 

given, — such a retina consists of neuropil substance and nerve-cells, 
as well as the retinal end-cells. In all such compound retinas, the 
retinal ganglion is not single, but two optic ganglia at least are 
included in it, so that there are two sets of nerve-cells and two 
synapses are always formed ; one between the retinal end-cells and 
the neurones of the first optic ganglion, which may be called the 
ganglion of the retina, the other between the first and second 
ganglia, which, seeing that the neuraxons of its cells form the 
optic nerve, may be called the ganglion of the optic nerve. The 
' neuropil ' formed by these synapses forms the molecular layers of the 
compound retina, and the cells themselves form the nuclear layers. 
Thus an upright compound retina, formed in the .same way as the 
upright simple retina, would be illustrated by Fig. 30. 



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THE EVIDENCE OF THE ORGANS OF VISION 73 

Further, in precisely the same way as in the case of the simple 
retina, such a compound retina may be upright or inverted. Thus, 
in the lateral eyes of crustaceans and insects, a compound retina of 
this kind is formed, which is upright ; while in the vertebrates the 
compound retina of the lateral eyes is inverted. 

The compound retina of vertebrates is usually described as com- 
posed of a series of layers, which may be analyzed into their several 
components as follows : — 

Layer of rods and cones \ 

External nuclear layer f retina proper } Ectodermic part 

External molecular layer { 

Internal nuclear layer > ganglion of retina 



Internal molecular layer { j retinal j neurodermic 

Optic nerve-cell layer 
Layer of optic nerve fibres 



Optic nerve-cell layer 1 ganglion of optic nerve j S^S 11011 > P* 1 * 

J 



The difference between the development of these two types of 
eye— those with a simple retina and those with a compound retina — 
has led, in the most natural manner, to the conception that the 
retina is developed, in the higher animals, sometimes from the cells of 
the peripheral epidermis, sometimes from the tissue of the brain — two 
modes of development termed by Balfour 'peripheral' and 'cerebral/ 
An historical survey of the question shows most conclusively that all 
investigators are agreed in ascribing the origin of the simple retina 
to the peripheral method of development, the retina being formed 
from the hypodermal cells by a process of invagination, while the 
cerebral type of development has been described only in the develop- 
ment of the compound retina. The natural conclusion from this fact 
is that, in watching the development of the compound retina, it is 
more difficult to differentiate the layers formed from the epidermal 
retinal cells and those formed from the epidermal optic ganglion- 
cells, than in the case of the simple retina, where the latter cells 
withdraw entirely from the surface. This is the conclusion to which 
Patten has come, and, indeed, judging from the text-book of Kor- 
schelt and Heider, it is the generally received opinion of the day 
that, as far as the Appendiculata are concerned, the retina, in the 
true sense — the retinal end-cells, with their cuticular rods, — is formed, 
in all cases, from the peripheral cells of the hypodermal layer, the 
cuticular rods being modifications of the general cuticular surface 
of the body. The apparent cerebral development of the crustacean 



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74 THE ORIGIN OF VERTEBRATES 

retina, as quoted from Bobretsky by Balfour, is therefore in reality 
the development of the retinal ganglion, and not of the retina proper. 
There is, I imagine, a universal belief that the natural mode of 
origin of a sense-organ, such as the eye, must always have been from 
the cells forming the external surface of the animal, and that direct 
origin from the central nervous system is a priori most improbable. 
It is, therefore, a matter of satisfaction to find that the evidence for 
the latter origin has universally broken down, with the single 
exception of the eyes of vertebrates and their degenerated allies ; a 
fact which points strongly to the probability that a reconsideration 
of the evidence upon which the present teaching of the origin of the 
vertebrate eye is based will show that here, too, a confusion has 
arisen between that part formed from the epidermal surface and that 
from the optic ganglion. 

The Median or Pineal Eyes. 

Undoubtedly, in recent times, the most important clue to the 
ancestry of vertebrates has been given by the discovery that the 
so-called pineal gland in the vertebrate brain is all that remains of a 
pair of median or pineal eyes, the existence of which is manifest in 
the earliest vertebrates ; so that the vertebrate, when it first arose, 
possessed a pair of median eyes as well as a pair of lateral eyes. 
The ancestor of the vertebrate, therefore, must also have possessed a 
pair of median eyes as well as a pair of lateral eyes. 

Very instructive, indeed, is the evidence with regard to these 
median eyes, for one of the great characteristics of the ancient 
palfeostracan forms is the invariable presence of a pair of median 
eyes as well as a pair of lateral eyes. In the living representative of 
such forms — Limulus — the pair of median eyes (Fig. 5) is well 
shown, and it is significant that here, according to Lankester and 
Bourne, these eyes are already in a condition of degeneration ; so 
also in many of the Palaeostraca (Fig. 7) the lateral eyes are the large, 
well- developed eyes, while the median eyes resemble those of Limulus 
in their insignificance. 

We see, then, that in the dominant arthropod race at the time 
when the fishes first appeared, the type of eyes consisted of a pair of 
well-developed lateral eyes and a pair of insignificant, partially 
degenerated, median eyes. Further, according to all paleontologists, 

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THE EVIDENCE OF THE ORGANS OF VISION 75 

in the best-preserved head-shields of the most ancient fishes, 
especially well seen in the Osteostraci, in Cephalaspis, Tremataspis, 
Auchenaspis, Keraspis, a pair of large, prominent lateral eyes existed, 
between which, in the mid-line, are seen a pair of small, insignificant 
median eyes. 

The evidence of the rocks, therefore, proves that the pair of 
median eyes which were originally the principal eyes (Hauptaugen), 
had already, in the dominant arthropod group been supplanted by 
a pair of lateral eyes, and had, in consequence, become small and 
insignificant, at the time when vertebrates first appeared. This dwind- 
ling process thus initiated in the arthropod itself has steadily continued 
ever since through the whole development of the vertebrates, with the 
result that, in the highest vertebrates, these median or pineal eyes 
have become converted into the pineal gland with its ' brain-sand/ 

In the earliest vertebrate these median eyes may have been 
functional ; they certainly were more conspicuous than in later forms. 
Alone among living vertebrates the right median eye of Ammocoetes 
is so perfect and the skin covering it so transparent that I have 
always felt doubtful whether it may not be of use to the animal, 
especially when one takes into consideration the undeveloped state 
of the lateral eyes in this animal, hidden as they are under the skin. 
Thus the one living vertebrate which is comparable with these 
extinct fishes is the one in which one of the pineal eyes is most well 
defined, most nearly functional. 

Before passing to the consideration of the structure of the 
median eyes of Ammocoetes, it is advisable to see whether these 
median eyes in other animals, such as arachnids and crustaceans, 
belong to any particular type of eyes, for then assuredly the median 
eyes of Ammocoetes ought to belong to the same type if they are 
derived from them. 

In the specialized crustacean, as in the specialized vertebrate, the 
median eyes have disappeared, at all events in the adult, but still 
exist in the primitive forms, such as Branchipus, which resemble the 
trilobites in some respects. On the other hand, the median eyes 
have persisted, and are well developed in the arachnids, both 
scorpions and spiders possessing a well- developed pair. The cha- 
racteristics of the median eyes must then be especially sought for in 
the arachnid group. 

Both scorpions and spiders possess many eyes, of which two are 



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76 THE ORIGIN OF VERTEBRATES 

always separate and median in position, while the others form lateral 
groups ; all these eyes possess a simple retina and a simple corneal 
lens. Grenacher was the first to point out that in the spiders two 
very distinct types of eye are found. In the one the retina is up- 
right ; in the other the retina is inverted, and the eye possesses a 
tapetal layer. The distribution of these two types is most suggestive, 
for the inverted retina is always found in the lateral eyes, never in 
the two median eyes ; these always possess a simple upright retina. 

In the crustaceans, the lateral eyes differ also from the median 
eyes, but not in the same way as in the arachnids ; for here both 
types of eye possess an upright retina, but the retina of the lateral 
eyes is compound, while that of the median eyes is simple. In other 
words, the median eyes are in all cases eyes with a simple upright 
retina and a simple cuticular lens, while the retina of the lateral 
eyes is compound or may be inverted, according as the animal 
in question possesses crustacean or arachnid affinities. The lateral 
eye of the vertebrate, possessing, as it does, an inverted compound 
retina, indicates that the vertebrate arose from a stock which was 
neither arachnid nor crustacean, but gave rise to both groups — in fact, 
was a member of the great palaeostracan group. What, then, is the 
nature of the median eyes in the vertebrate ? 

The Median Eyes of Ammoccetes. 

The evidence of Ammoccetes is so conclusive that I, for one, can- 
not conceive how it is possible for any zoologist to doubt whether 
the parietal organ, as they insist on calling it, had ever been an eye, 
or rather a pair of eyes. 

Any one who examines the head of the larval lamprey will see 
on the dorsal side, in the median line, first, a somewhat circular orifice 
— the unpaired nasal opening ; and then, tailwards to this, a well- 
marked circular spot, where the skin is distinctly more transparent 
than elsewhere This spot coincides in position with the underlying 
dorsal pineal eye, which shines out conspicuously owing to the 
glistening whiteness of its pigment. Upon opening the brain- case 
the appearance as in Fig. 20 is seen, and the mass of the right ganglion 
habtnulce (G.H.B.), as it has been called, stands out conspicuously as 
well as the right or dorsal pineal eye (Pn.). Both eye and ganglion 
appear at first sight to be one-sided, but further examination shows 
that a left ganglion habenida is present, though much smaller than on 



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THE EVIDENCE OF THE ORGANS OF VISION 



77 



the right side. In connection with this is another eye-like organ — the 
left or ventral pineal eye, — much more aborted, much less like an eye 
than the dorsal one ; so also there are two bundles of peculiar fibres 




Fig. 81. — One of a Series op Horizontal Sections through the Head of 

Ammooetes. 

l.m.y upper lip muscles; w.c, muco-cartilage ; it., nose; na.c, nasal cartilage; pn. f 
right pineal eye and nerve; g.h.r., right ganglion habenultf ; s.m. t somatic 
muscles; cr. f membranous wall of cranium; ch. t choroid plexus; gl., glandular 
substance and pigment filling up brain-case. 

called Meynert's bundles, which connect this region with the infra- 
infundibular region of the brain ; of these, the right Meynert's bundle 



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78 



THE ORIGIN OF VERTEBRATES 



is much larger than the left. This difference between right and left 
indicates a greater degeneration on the left side, and points distinctly 
to a close relationship between the nerve-masses known as ganglia 
habenul.ce and the median eyes. In my opinion this ganglion is, in 
part, at all events, the optic ganglion of the median eye on each side. 
It is built up on the same type as the optic ganglia of invertebrate 





Fig. 32.— Eye op Acilius Larva, with Fio. 33.— Pineal Eye op Ammoccetes, 
its Optic Ganglion. with its Ganglion HabenuUp. 

On the right side the nerve end-cells On the left Hide the eye is drawn as it 
have been drawn free from pigment. appeared in the section. On the right 

side I have removed the pigment from 
the nerve end-cells, and drawn the eye 
as, in my opinion, it would appear if 
it were functional. 

simple eyes, with a cortex of small round cells and a medulla of fine 
nerve-fibres. Into this ganglion, on the right side, there passes a very 
well-defined nerve — the nerve of the dorsal eye. The eye itself with 
its nerve, pn., and its optic ganglion, g.h.r., is beautifully shown by 
means of a horizontal section through the head of Ammocoetes 
(Fig. 31). Originally, as described by Scott, the eye stood vertically 



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THE EVIDENCE OF THE ORGANS OF VISION 



79 



above its optic ganglion, and presented an appearance remarkably like 
Fig. 32, which represents one of the simple eyes and optic ganglia 
of a larva of Acilius as described by Patten ; then, with the forward 
growth of the upper lip, 

the right pineal eye was (^J^^""' n l 

dragged forward and its 
nerve pulled horizon- 
tally over the ganglion 
habenidce. For this 
reason the eye, nerve, 
and ganglion are better 
shown in a nearly hori- 
zontal than in a trans- 
verse section. 

The optic nerve be- 
longing to this eye is 
most evident and clearly 
shown in Fig. 31, and in 
the series of consecutive 
sections which follow 
upon this section; no 
doubt can arise as to 
the structure in ques- 
tion having been the 
nerve Qf the eye, even 
though, as is possible, it 
does not contain any 
functional nerve-fibres. 

The second, ventral 
or left, eye, belonging 
to the left ganglion 
habenuhe is very dif- 
ferent in appearance, 
being much less evi- 
dently an eye. Fig. 34 




Fig. 34.— Horizontal Section through Brain op 
Ammocostes, to show the Left, or Ventral 
Pineal Eye. 

pn. 21 left or ventral pineal eye ; pn. lt last remnant of 
right, or dorsal pineal eye ; g.h.r., right ganglion 
habenulcB; g.h.l. l% g.h.l. 3 , parts of left ganglion 
habenuke ; pi., fold oipia mater which separates 
the left ganglion habenuks from the left pineal 
eye ; /., strands of nerve-fibres connecting the 
left eye with its ganglion, g.h.l. 3 ; V 3f third 
ventricle ; Y.aq. $ ventricle of aquseduct. 



is one of the same 
series of horizontal sections as Fig. 31, pn.i being the last remnant 
of the right, or dorsal, eye, while pn. 2 shows the left, or ventral, eye 
with its connection with the left ganglion habenidce. 



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80 THE ORIGIN OF VERTEBRATES 

In a series of sections I have followed the nerve of the right pineal 
eye to its destination, as described in my paper in the Quarterly 
Journal of Microscopical Science, and have found that it enters into 
the ganglion habenulw just as the nerve to any simple eye enters 
into its optic ganglion. This nerve, as I have shown, is a very dis- 
tinct, well-de6ned nerve, with no admixture of ganglion-cells or of 
connective tissue, very different indeed to the connection between 
the left pineal eye and its optic ganglion. Here there is no defined 
nerve at all ; but the cells of the ganglion hdbenulm stretch right up to 
the remains of the eye itself. Seeing, then, that both the eye and 
ganglion on this side have reached a much further grade of degenera- 
tion than on the right side, it may be fairly concluded that the 
original condition of these two median eyes is more nearly repre- 
sented by the right eye, with its well-defined nerve and optic gang- 
lion, than by the left eye, or by the eyes in lizards and other animals 
which do not show so well-defined a nerve as is possessed by 
Ammoccetes. Quite recently Dendy has examined the two median 
eyes in the New Zealand lamprey Geotria australis. In this species 
the second eye is much better defined than in the European lamprey, 
and its connection with the ganglion hdbenulm is more nerve-like. 
In neither eye, however, is the nerve so clean cut and isolated as is the 
nerve of the dorsal, or right, eye in the Ammoccetes stage of Pctromy- 
zonPlaneri; in both, cells resembling those of the cortex of the 
ganglion habenulm and connective tissues are mixed up with the 
nerve-fibres which pass from each eye to its respective optic ganglion. 

The Eight Pineal Eye of Ammoccetes. 

The optic fibres of the right median eye of Ammoccetes are con- 
nected with a well-defined retina, the limits of which are defined 
by the white pigment so characteristic of this eye. This pigment is 
apparently calcium phosphate, which still remains as the ' brain-sand * 
of the human pineal gland. The cells, which are hidden by this pig- 
ment, were described by me in 1890 as the retinal end-cells with large 
nuclei. In 1893, Studnifka examined them more closely, and con- 
cluded that the retinal cells are of two kinds : the one, nerve end-cells, 
the sensory cells proper ; the other, pigmented epithelial cells, which 
surround the sense- cells. The sense-cells may contain some of the 
white pigment, but not so much as the other cells. Similarly, in the 



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THE EVIDENCE OF THE ORGANS OF VISION 8 1 

median eyes of Limulus, Lankester and Bourne find it difficult to 
determine how far the retinal end-cells contain pigment and how far 
that pigment really is in the cells surrounding these nerve end-cells. 

The interior of the eye presents the appearance of a cavity in 
shape like a cornucopia, the stalk of which terminates at the place 
where the nerve enters. This cavity is not empty, but the posterior 
part of it is filled with the termination of the nerve end-cells of the 
retina, as pointed out by me and confirmed by Studnipka. These 
terminations are free from pigment, and contain strikingly trans- 
lucent bodies, which I have described in my paper in the Quarterly 
Journal, and called rhabdites, for they present the same appearance 
and are situated in the same position as are many of the rhabdites 
on the terminations of the retinal end-cells of arthropod eyes. 
Studni$ka has also seen these appearances, and figures them in 
his second paper on the nerve end-cells of the pineal eye of 
Ammoccetes. 

Up to this point the following conclusions may be drawn : — 

1. Ammoccetes possesses a pair of median eyes, just as was the 

case with the most ancient fishes, and with the members of 
the contemporary paheostracan group. 

2. The retina of one of these eyes is well-defined and upright, 

not inverted, and therefore in this respect agrees with that 
of all median eyes. 

3. The presence of nerve end-cells, with pigment either in them or 

in cells around them, to the unpigmented ends of which trans- 
lucent bodies resembling rhabdites are attached, is another 
proof that this retina agrees with that of the median eyes of 
arthropods. 

4. The simple nature of the nerve with its termination in an 

optic ganglion closely resembling in structure an arthropod 

optic ganglion, together with Studnipka's statement that the 

nerve end-cells pass directly into the nerve, points directly 

to the conclusion that this retina is a simple, not a compound, 

retina, and that it therefore in this respect also agrees with 

the retina of all median eyes. 

With respect to this last conclusion, neither I myself nor 

Studni^ka have been able to see any definite groups of cells 

between the nerve end-cells and the optic nerve such as a compound 

retina necessitates. 

o 



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82 THE ORIGIN OF VERTEBRATES 

On the other hand, Dendy describes in the New Zealand lamprey, 
Geotria atistralis, a cavity where the nerve enters into the eye, 
which he calls the atrium. This cavity is distinct from the main 
cavity of the eye, and is separated from it by a mass of cells similar 
in appearance to those of the cortex of the ganglion Juibenulce. In 
these two eyes then, groups of cells, resembling in appearance those 
belonging to an optic ganglion, exist in the eyes themselves. This 
atrium is evidently that part of the central cavity which I have 
called the handle of the cornucopia in the European lamprey, and 
the very fact that it is separated from the rest of the central cavity 
is evidence that we are dealing here with a later stage in the history 
of the pineal eyes than in the case of the Ammocoetes of Petromyzon 
Planeri. Taking also into consideration the continuity of the mass 
of small ganglion-cells which surround this atrium with the cells of 
the ganglion habenulce by means of the similar cells scattered along 
the course of the nerve, and also bearing in mind the fact already 
stated that in the more degenerate left eye of Ammocoetes the cells 
of the ganglion habenulce extend right up to the eye itself, it seems 
more likely than not that these cells do not represent the original 
optic ganglion of a compound retina, but rather the subsequent 
invasion, by way of the pineal nerve, of ganglion-cells belonging to 
a portion of the brain. In the last chapter it has been suggested 
that the presence of the trochlear or fourth cranial nerve has given 
rise to the formation of the cerebellum by a similar spreading. 

There is certainly no appearance in the least resembling a 
compound retina such as is seen in the vertebrate or crustacean 
lateral eye. In the median eyes of scorpions and of Limulus, cells 
are found within the capsule of the eye among the nerve-fibres and 
the nerve end-cells. These are especially numerous in the median 
eyes of Limulus, as described by Lankester and Bourne, and are 
called by them intrusive connective tissue cells. The meaning of 
these cells is not, to my mind, yet settled. It is sufficient for my 
purpose to point out that the presence of cells interneural in position 
among the nerve end-cells of the retina of the median eyes of 
Ammocoetes is more probable than not, on the assumption that the 
retina of these eyes is built up on the same plan as that of the 
median eyes in Limulus and the scorpions. 

It is further to be borne in mind that these specimens of Geotria 
worked at by Dendy were in the ' Velasia ' stage of the New Zealand 



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THE EVIDENCE OF THE ORGANS OF VISION 



83 



lamprey, and correspond, therefore, more nearly to the Petromyzon 
than to the Ammocoetes stage of the European lamprey. 

The Dioptric Apparatus. 

Besides the retina, all eyes possess a dioptric apparatus. What 
is the evidence as to its nature in these vertebrate median eyes? 
Lankester and Bourne have divided the eyes of scorpions and 

I 




Fig. 35. — Eye op Acilius Lakva:. (After Patten.) 

l. t chitinous lens; c, corneagen ; pr., prc-retinal layer ; rh. f rhabdites ; rd., retinal 

end-cells. 

Limulus into two kinds, monostichous and diplostichous. In the 
first the retinal cells are supposed to give rise to not only rhabdites 
but also the cuticular chitinous lens, so that the eye is one-layered ; 
in the second the lens is formed by a well-marked hypodermal layer, 
in front of the retina, composed of elongated cells, so that these eyes 
are two-layered or diplostichous. The lateral eyes, according to 
them, are all monostichous, but the median eyes are diplostichous. 
This distinction is not considered valid by other observers. Thus, 



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8 4 



THE ORIGIN OF VERTEBRATES 




as already indicated, Patten looks on all these eyes as three-layered, 
and states that in all cases a corneagen or vitreogen layer exists, 
which gives origin to the lens. For my own part I agree with 

Patten, but we are not con- 
z • cerned here with the lateral 

eyes. It is sufficient to note 
that all observers are agreed 
that the median eyes are 
characterized by this well - 
marked cell-layer, the so-called 
vitreous or corneagen layer of 
cells. 

This layer (c. t Fig. 35) is 
composed of much - elongated 
cells of the hypodermal layer, 
in each of which the large 
nucleus is always situated to- 
wards the base of the cell. 
The space between it and the 
retina contains, according to 
Patten the cells of the pre- 
retinal layer {pr.). These may be so few and insignificant as to give 
the impression that the vitreous layer is immediately adjacent to the 
retina {ret.). 

Let us turn now to the right pineal eye of Ammocoetes (Fig. 37) 
and see what its further structure is. The anterior part of the eye 
is free from pigment, and is composed, as is seen in hematoxylin or 
carmine specimens, of an inner layer of nuclei which are frequently 
arranged in a wavy line. From this nucleated layer, strands of tissue, 
free from nuclei, pass to the anterior edge of the eye. 

In the horizontal longitudinal sections it is seen that these strands 
are confined to the middle of the eye. On each side of them the 
nuclear layer reaches the periphery, so that if we consider these 
strands to represent long cylindrical cells, as described by Beard, 
then the anterior wall may be described as consisting of long 
cylindrical cells, which are flanked on either side by shorter cells of 
a similar kind. The nuclei at the base of these cylindrical cells are 
not all alike. We see, in the first place, large nuclei resembling the 
large nuclei belonging to the nerve end-cells ; these are the nuclei of 



Fig. 36. — Eye op Hydrophilus Larva, 
with the Pigment over the Retinal 

END-CELL8. 

l. $ chitinous lens; c, corneagen; pr., pre- 
retinal layer; rft., rhabdites; ret., retinal 
end-cells. 



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THE EVIDENCE OF THE ORGANS OF VISION 



85 



the long cylindrical cells. We see also smaller nuclei in among 
these larger ones, which look like nuclei of intrusive connective 
tissue, or may perhaps form a distinct layer of cells, situated between 
the cells of the anterior wall and the terminations of the nerve 
end-cells already referred to. 

These elongated cells are in exactly the same position and present 
the same appearance as the cells of the corneagen layer of any median 
eye. Like the latter they are ___ 
free from pigment and never 
show with osmic staining any 
sign of the presence of trans- 
lucent rhabdite - like bodies, 
such as are seen in the termi- 
nation of the retinal cells, and 
like the latter their nuclei are 
at the base. The resemblance 
between this layer and the 
corneagen cells of any median 
eye is absolute. Between it 
and the terminations of the 
retinal cells there exists some 
ill-defined material certainly 
containing cells which may 
well correspond to Patten's 
pre-retinal layer of cells. 

Eetina, corneagen, nerve, 
optic ganglion, all are there, all 
in their right position, all of 
the right structure, what more 
is needed to complete the 
picture ? 

In ordef to complete the dioptric apparatus a lens is necessary. 
Where, then, is the lens in these pineal eyes ? In all the arachnid eyes, 
whether median or lateral, the lens is a single corneal lens composed 
of the external cuticle, which is thickened over the corneagen cells. 
This thickened cuticle is composed of chitin, and is not cellular, 
but is dead material formed out of the living underlying corneagen 
cells. Such a lens is in marked contrast to the lens of the lateral 
vertebrate eye, which is formed by living cells themselves. This 








Fia. 37.— Pineal Eye of Ammoccetes, 
with its Ganglion JlabenuUr. 



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86 THE ORIGIN OF VERTEBRATES 

thickening of the cuticular layer to form a lens could only exist as 
long as that layer is absolutely external, so that the light strikes 
immediately upon it ; for, if from any cause the eye became situated 
internally, the place of such a lens must be filled by the structures 
situated between it and the surface, and the thickened cuticle would 
no longer be formed. 

In all vertebrates these pineal eyes are separated from the 
external surface by a greater or less thickness of tissues ; in the 
case of Ammoccetes, as is* seen in Fig. 31, the eye lies within the 
membranous cranial wall, and is attached closely to it. The position, 
then, of the cuticular, or corneal lens, as it is often called, on the 
supposition that this is a median eye of the arachnid type, is taken 
by the membranous cranium, and, as I have described in my 
paper in the Quarterly Journal, on carefully lifting the eye in the 
fresh condition from the cranial wall, it can be seen under a 
dissecting microscope that the cranial wall often forms at this 
spot a lens-like bulging, which fits the shallow concavity of the 
surface of the eye, and requires some little force to separate it from 
the eye. 

As will appear in a subsequent chapter, this cranial wall has 
been formed by the growth, laterally and dorsally, of a skeletal 
structure known by the name of the plastron. The last part of it to 
be completed would be that part in the mid-dorsal line, where appa- 
rently, in consequence of the insinking of the degenerating eyes, a 
dermal and subdermal layer already intervened between the source 
of light and the eyes themselves. 

When the membranous cranium was completed in the mid-dorsal 
region, it was situated here as elsewhere just internally to the sub- 
dermal layer, and therefore enclosed the pineal eyes. This, to my 
mind, is the reason why the pineal eyes, which, in all other respects, 
conform to the type of the median eyes of an arachnid-like animal, 
do not possess a cuticular lens. Other observers have attempted to 
make a lens out of the elongated cells of the anterior wall of the 
eye (my corneagen layer), but without success. 

Studnifka, who calls this layer the pellu^ida, does not look upon 
it as the lens, but considers, strangely enough, that the translucent 
appearances at the ends of each nerve end-cell represent a lens for 
that cell, so that every nerve end-cell has its own lens. Still more 
strange is it that, holding this view, he should yet consider these knobs 



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THE EVIDENCE OF THE ORGANS OF VISION 87 

to be joined by filaments to the cells in the anterior wall of the eye, 
a conception fatal to the action of such knobs as lenses. 

The discovery that the vertebrate possesses, in addition to the 
lateral eyes, a pair of median eyes, which are most conspicuous in 
the lowest living vertebrate, together with the fact that such eyes 
are built up on the same plan as the median eyes of living crus- 
taceans or arachnids, not only with respect to the eye itself but also 
to its nerve and optic ganglion, constitutes a fact of the very greatest 
importance for any theory of the origin of vertebrates ; especially in 
view of the further fact, that similar eyes in the same position are 
found not only in all the members of the Palajostraca, but also in all 
those ancient forms (classed as fishes) which lived at that time. At 
one and the same moment it proves the utter impossibility of 
reversing dorsal and ventral surfaces, points in the very strongest 
manner to the origin of the vertebrate from some member or other 
of the palaeostracan group, and insists that the advocates of the 
origin of vertebrates from the Heraichordata, etc., should give an 
explanation of the presence of these two median eyes of a more con- 
vincing character than that given here. 

The Lateral Eyes. 

Turning now to the consideration of the lateral eyes, we see that 
these eyes in the arachnids often possess an inverted retina, in the 
crustaceans always an upright retina. In the arachnids they possess 
a simple retina, while in the crustaceans their retina is compound ; 
so that in the latter ca9e the so-called optic nerve is in reality a 
tract of fibres connecting together the brain-region with a variable 
number of optic ganglia, which have been left at the periphery in 
close contact with the retinal cells, when the brain sunk away from 
the superficial epithelial covering. 

There is, then, this difference between the lateral eyes of crus- 
taceans and arachnids, that the retina of the former is compound, but 
never inverted, while that of the latter may be inverted, but is 
always simple. 

The retina of the lateral eyes of the vertebrate resembles both of 
these, for it is compound, as in the crustacean, and inverted as in 
the arachnid. 

It must always be borne in mind that in the palaeostracan epoch 



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88 THE ORIGIN OF VERTEBRATES 

the dominant race was neither crustacean nor arachnid, but partook 
of the characters of both; also, as is characteristic of dominance, 
there was very great variety of form, so that it seems more probable 
than not that some of these forms may have combined the arachnid 
and crustacean characteristics to the extent of possessing lateral eyes 
with an inverted yet compound retina. A certain amount of 
evidence points in this direction. As already stated, the compound 
retina which characterizes the vertebrate lateral eyes is character- 
istic of all facetted eyes, and in the trilobites facetted lateral eyes 
are commonly found. From this it may be concluded that many of 
the trilobites possessed eyes with a compound retina. There have, 
however, been found in certain species, e.g. Harpes vittalus and 
Harpes ungula, lateral eyes which were not facetted, and are believed 
by Korschelt and Heider to be of an arachnid nature. They say, 
" Palaeontologists have appropriately described them as ocelli, 
although, from a zoological point of view, they do not deserve this 
name, having most probably arisen in a way similar to that con- 
jectured in connection with the lateral eyes of scorpions." If this 
conjecture is right, then in these forms the retina may have been 
inverted, but because they belonged to the trilobite group, the retina 
was most probably compound, so that here we may have had the 
combination of the arachnid and crustacean characteristics. On the 
other hand,, in some forms of Branchipus, and many of the Gamma- 
ridae, a single corneal lens is found in conjunction with an eye of the 
crustacean type, so that a non-facetted lateral eye, found in a fossil 
form, would not necessarily imply the arachnid type of eye with the 
possibility of an inverted retina. Whatever may be the ultimate 
decision upon these particular forms, the striking fact remains, that 
both in the vertebrate and in the arachnid the median eyes possess 
a simple upright retina, while the lateral eyes possess an inverted 
retina, and that both in the vertebrate and the crustacean the 
median eyes possess a simple upright retina, while the lateral eyes 
possess a compound retina. 

The resemblance of the retina of the lateral eyes of vertebrates 
to that of the lateral eyes of many arthropods, especially crustaceans, 
has been pointed out by nearly every one who has worked at these 
invertebrate lateral eyes. The foundation of our knowledge of the 
compound retina is Berger's well-known paper, the results of which 
are summed up by him in the following two main conclusions. 



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THE EVIDENCE OF THE ORGANS OF VISION 



8 9 



1. The optic ganglion of the Arthropoda consists of two parts, of 
which the one stands in direct inseparable connection with the 
facetted eye, and together with the layer of retinal rods forms the 
retina of the facetted eye, while the other part is connected rather 
with the brain, and is to be considered as an integral part of the 
brain in the narrower sense of the word. 

2. In all arthropods examined by him, the retina consists of five 
layers, as follows : — 

(1) The layer of rods and their nuclei. 

(2) The layer of nerve-bundles. 

(3) The nuclear layer. 

(4) The molecular layer. 
(f>) The ganglion cell layer. 

Berger passes under review the structure and arrangement of 
the optic ganglion in a large number of different groups of arthropods, 
and concludes that in 
all cases one part of 
the optic ganglion is 
always closely attached 
to the visual end-cells, 
and this combination 
he calls the retina. 
On the other hand, the 
nerve-fibres which con- 
nect the peripheral part 
of the optic ganglion 
with the brain, the so- 
called optic nerve, are 
by no means homolo- 
gous in the different 
groups; for in some 
cases, as in many of 
the stalk-eyed crusta- 
ceans, the whole optic 
ganglion is at the pe- 
riphery, while in others, as in the Diptera, only the retinal ganglion 
is at the periphery, and the nerve-stalk connects this with the rest 
of the optic ganglion, the latter being fused with the main brain- 
mass. In the Diptera, in fact, according to Berger, the optic nerve 




Fig. 38.— The Retina of Musca. (After Berger.) 

Br., brain ; 0.n.> optic nerve ; n.l.o.g., nuclear layer of 
ganglion of optic nerve; m.L, molecular layer 
(Punktsubstanz) ; n.l.r.g.i. and n.l.r.g.o., inner and 
outer nuclear layers of the ganglion of the retina ; 
/.fcr.r., terminal fibre-layer of retina; r., layer of 
retinal end-cells (indicated only). 



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go 



THE ORIGIN OF VERTEBRATES 



and retina are most nearly comparable to those of the vertebrate. 
For this reason I give Berger's picture of the retina of Musea 
(Fig. 38), in order to show the arrangement there of the retinal 
layers. 

In Branchipus and other primitive Crustacea, Berger also finds 
the same retinal layers, but is unable to distinguish in the brain the 
rest of the optic ganglion. Judging from Berger's description of 
Branchipus, and Bellonci's of Sphseroma, it would almost appear 
as though the cerebral part of the retina in the higher forms 
originated from two ganglionic enlargements, an external and 



Sup. Segment Ant 1 




Fia. 89.— The Brain op Sphceroma serratum. (After Bellonci.) 

Ant. I. and Ant. II., nerves to 1st and 2nd antennae, f.br.r., terminal fibre-layer of 
retina ; Op. g. I., first optic ganglion ; Op. g. II., second optic ganglion ; O.n., 
optic nerve-fibres forming an optic chiasma. 

internal enlargement, as Bellonci calls them. The external ganglion 
(Op. g. Z, Fig. 39) may be called the ganglion of the retina, the cells 
of which form the nuclear layer of the higher forms, and the internal 
ganglion (Op. g. II., Fig. 39), from which the optic nerve-fibres to the 
brain arise, may therefore be called the ganglion of the optic nerve. 
Bellonci describes how in this latter ganglion cells are found very 
different to the small ones of the external ganglion or ganglion of 
the retina. So also in Branchipus, judging from the pictures of 
Berger, Claus, and from my own observations (cf. Fig. 46, in which 
the double nature of the retinal ganglion is indicated), the peripheral 
part of the optic ganglion — i.e. the retinal ganglion — may be spoken 



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THE EVIDENCE OF THE ORGANS OF VISION 



91 



nl.r.g, 



of as composed of two ganglia. The external of these is clearly the 
ganglion of the retina ; its cells form the nuclear layer, the striking 
character of which, and close resemblance to the corresponding layer 
in vertebrates, is shown by Claus' picture, which I reproduce (Fig. 40). 
The internal ganglion with which the optic nerve is in connection 
contains large ganglion cells, which, to- 
gether with smaller ones, form the gang- 
lionic layer of Berger. 

The most recent observations of the 
structure of the compound retina of the 
crustacean eye are those of Parker, who, 
by the use of the methylene blue method, 
and Golgi's method of staining, has been 
able to follow out the structure of the 
compound retina in the arthropod on the 
same lines as had already been done for 
the vertebrate. These two methods have 
led to the conclusion that the arthropod 
central nervous system and the verte- 
brate central nervous system are built up 
in the same manner — viz. by means of a 
series of ganglia connected together, each 
ganglion being composed of nerve-cells, 
nerve-fibres, and a fine reticulated sub- 
stance called by Leydig in arthropods 
1 Punktsubstanz/ and known in verte- 
brates and in invertebrates at the present 
time as 'neuropil.' A further analysis 
resolves the whole system into a combi- 
nation of groups of neurones, the cells 
and fibres of which form the cells and 
fibres of the ganglia, while their dendritic 
connections with the terminations of other neurones, together with 
the neuroglia-cells form the 'neuropil/ As is natural to expect, 
that part of the central nervous system which helps to form the 
compound retina is built up in the same manner as the rest of the 
central nervous system. 

Thus, according to Parker, the mass of nervous tissue which 
occupies the central part of the optic stalk in Astacus is composed 




Fig. 40.— Bipolar Cells of 
Nuclear Layer in Retina 
of Branchipus. (After 
Claus.) 

/.6r.r., terminal fibre - layer 
of retina; n.l.r.g., bipolar 
cells of the ganglion of the 
retina = inner nuclear layer ; 
m.L, Punktsubstanz = inner 
molecular layer ; 6.m M base- 
ment membrane formed by 
neurilemma round central 
nervous svstem. 



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92 THE ORIGIN OF VERTEBRATES 

of four distinct ganglia; the retina is connected with the first of 
these by means of the retinal fibres, and the optic nerve extends 
proximally from the fourth ganglion to the brain. Each ganglion con- 
sists of ganglion-cells, nerve-fibres, and ' neuropil/ and, in addition, 
supporting cells of a neuroglial type. By means of the methylene 
blue method and the Golgi method, it is seen that the retinal end- 
cells, with their visual rods, are connected with the fibres of the 
optic nerve by means of a system of neurones, the synapses of 
which take place in and help to form the ' neuropil ' of the various 
ganglia. Thus, an impulse in passing from the retina to the brain 
would ordinarily travel over five neurones, beginning with one of 
the first order and ending with one of the fifth. He makes five 
neurones although there are only four ganglia, because he reckons 
the retinal cell with its elongated fibre as a neurone of the first 
order, such fibre terminating in dendritic processes which form 
synapses in the ' neuropil ' of the first ganglion with the neurones of 
the second order. 

Similarly the neurones of the second order terminate in the 
' neuropil ' of the second ganglion, and so on, until we reach the 
neurones of the fifth order, which terminate on the one hand in the 
1 neuropil ' of the fourth ganglion, and on the other pass to the optic 
lobes of the brain by their long neuraxons — the fibres of the optic 
nerve. 

He compares this arrangement with that of Branchipus, Apus, 
Estheria, Daphnia, etc, and shows that in the more primitive 
crustaceans the peripheral optic apparatus was composed, not of 
four but of two optic ganglia, not, therefore, of five but of three 
neurones, viz. — 

1. The neurone of the first order — i.e. the retinal cell with its 
fibre terminating in the ' neuropil ' of the first optic ganglion (ganglion 
of the retina). 

2. The neurone of the second order, which terminates in the 
' neuropil ' of the second ganglion (ganglion of the optic nerve). 

3. The neurone of the third order, which terminates in the optic 
lobes of the brain by means of its neuraxons (the optic nerve). 

We see, then, that the most recent researches agree with the 
older ones of Berger, Claus, and Bellonci, in picturing the retina of 
the primitive crustacean forms as formed of two ganglia only, and 
not of four, as in the specialized crustacean group the Malacostraca. 



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THE EVIDENCE OF THE ORGANS OF VISION 93 

The comparison of the arthropod compound retina with that of 
the vertebrate shows, as one would expect upon the theory of the 
origin of vertebrates put forward in this book, that the latter retina 
is built up of two ganglia, as in the more primitive less specialized 
crustacean forms. The modern description of the vertebrate retina, 
based upon the Golgi method of staining, is exactly Parker's descrip- 
tion of the simpler form of crustacean retina in which the ' neuropil ' 
of the first ganglion is represented by the external molecular 
layer, and that of the second ganglion by the internal molecular 
layer; the three sets of neurones being, according to Parker's 
terminology : — 

1. The neurones of the first order — viz. the visual cells — the 
nuclei of which form the external nuclear layer, and their long 
attenuated processes form synapses in the external molecular layer 
with 

2. The neurones of the second order, the cells of which form the 
internal nuclear layer, and their processes form synapses in the 
internal molecular layer with 

3. The neurones of the third order, the cells of which form the 
ganglionic layer and their neuraxons constitute the fibres of the optic 
nerve which end in the optic lobes of the brain. 

Strictly speaking, of course, the visual cells with their elongated 
processes have no right to be called neurones : I only use Parker's 
phraseology in order to show how closely the two retinas agree even 
to the formation of synapses between the fine drawn-out processes of 
the visual cells and the neurones of the ganglion of the retina. 

The Ketina of the Lateral Eye of Ammoccetes. 

As in the case of all other organs, it follows that if we are dealing 
here with a true genetic relationship, then the lower we go in the 
vertebrate kingdom the more nearly ought the structure of the retina 
to approach the arthropod type. It is therefore a matter of intense 
interest to determine the nature of the retina in Ammoccjetes in order 
to see whether it differs from that of the higher vertebrates, and if 
so, whether such differences are explicable by reference to the structure 
of the arthropod eye. 

Before describing the structure of this retina it is necessary to 
clear away a remarkable misconception, shared among others by 



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94 THE ORIGIN OF VERTEBRATES 

Balfour, that this eye is an aborted eye, and that it cannot be 
considered as a primitive type. Thus Balfour says: "Considering 
the degraded character of the Ammoccete eye, evidence derived from 
its structure must be received with caution," and later on, "the most 
interesting cases of partial degeneration are those of Myxine and the 
Ammocoete. The development of such aborted eyes has as yet been 
studied only in the Ammoccete, in which it resembles in most 
important features that of other Vertebrata." 

Again and again the aborted character of the eye is stated to be 
evidence of degeneration in the case of the lamprey. What such a 
statement means, why the eye is in any way to be considered as 
aborted, is to me a matter of absolute wonderment : it is true that 
in the larval form it lies under the skin, but it is equally true that 
at transformation it comes to the surface, and is most evidently as 
perfect an eye as could be desired. There is not the slightest sign 
of any degeneration or abortion, but simply of normal development, 
which takes a longer time than usual, lasting as it does throughout 
the life-time of the larval form. 

Kohl, who has especially studied degenerated vertebrate eyes, 
discusses with considerable fulness the question of the Ammocoetes 
eye, and concludes that in aborted eyes a retarded development 
occurs, and this applies on the whole to Ammocoetes, " but with the 
important difference that in this case the period of retarded develop- 
ment is not followed by a stoppage, but on the contrary by a period 
of very highly intensified progressive development during the meta- 
morphosis," with the result that "the adult eye of Petromyzon 
Planeri does not diverge from the ordinary type." 

Referring in his summing up to this retarded development, he 
says: "Such reminiscences of embryonic conditions are after all 
present here and there in normally developed organs, and by no 
means entitle us to speak of abnormal development." 

The evidence, then, is quite clear that the eye of Petromyzon, 
or, indeed, of the full-grown Ammocoetes, is in no sense an abnormal 
eye, but simply that its development is slow during the ammoccete 
stage. The retina of Petromyzon was figured and described by 
Langerhans in 1873. He describes it as composed of the following 
layers : — 

(1) Menibrana limitans interna. 

(2) Thick inner molecular layer. 



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THE EVIDENCE OF THE ORGANS OF VISION 



95 



(3) Optic fibre layer. 

(4) Thick inner nuclear layer. 

(5) Peculiar double-layered ganglionic layer. 

(6) External molecular layer. 

(7) External nuclear layer. 

(8) Membrana limitans externa. 

(9) Layer of rods. 

(10) Pigment-epithelium. 
He points out especially the peculiarity of layer (2) (2, Fig. 41), the 
inner molecular, in which two rows of nuclei are arranged with great 




m 



Fig. 41. — Retina and Optic Nerve of Petromyzon. (After Muller and 

Langerhans.) 

On the left side the Miillerian fibres and pigment-epithelium are represented alone. 
The retina is divided into an epithelial part, C (the layer of visual rod -cells), and 
a neurodermal or cerebral part which is formed of, A, the ganglion of the optic 
nerve and, B, the ganglion of the retina. 1, int. limiting membrane ; 2, int. 
molecular layer with its two layers of cells ; 8, layer of optic nerve fibres ; 4, int. 
nuclear layer ; 5, double row of tangential fulcrum cells ; 6, layer of terminal 
retinal fibres; 7, ext. nuclear layer; 8, ext. limiting membrane; 9, layer of 
rods ; 10, layer of pigment-epithelium. Z>, axial cell layer (Axenstrang) in optic 
nerve. • The layer 6 is drawn rather too thick. 

regularity, the one row closely touching the membrana limitans 
interna, the other at the inner boundary of the middle third of the 



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96 THE ORIGIN OF VERTEBRATES 

molecular layer. Of these two rows of nuclei, he describes the inner- 
most as composed almost entirely of large nuclei belonging to ganglion 
cells, while the outermost is composed mainly of distinctly smaller 
nuclei, which in staining and appearance appear to belong not to 
nerve-cells but to the true reticular tissue of the molecular layer. 

He also draws special attention to the remarkable layer (5) (5, 
Fig. 41), which is not found in the retina of the higher vertebrates, 
the cells of which, in his opinion, are of the nature of ganglion-cells. 

W. Miiiler, in 1874, gave a most careful description of the eye 
of Ammocoetes and Petromyzon, and traced the development of the 
retina; the subsequent paper of Kohl does not add anything new, 
and his drawings are manifestly diagrams, and do not represent the 
appearances so accurately as Miiller's illustrations. In the 
accompanying figure (Fig. 41) I reproduce on the right-hand side 
Miiller's picture of the retina of Petromyzon, but have drawn it, as 
in Langerhans' picture, at the place of entry of the optic nerve. 

From his comparison of this retina with a large number of other 
vertebrate retinas, he comes to the conclusion that the retina of all 
vertebrates is divisible into 

A, An ectodermal (epithelial) part consisting of the layer of the 

visual cells, and 

B. A neurodermal (cerebral) part which forms the rest of the 

retina. 
Further, MUller points out that the neuroderm gives origin through- 
out the central nervous system to two totally different structures, on 
the one hand to the true nervous elements, on the other to a system 
of supporting cells and fibres which cannot be classed as connective 
tissue, for they do not arise from mesoblast, and are therefore called 
by him ' fulcrum-cells/ In the retina he recognizes two distinct 
groups of such supporting structures — (1) a system of radial fibres 
with well-marked elongated nuclei, which extend between the two 
limiting layers, and form at their outer ends a membrane-like 
expansion which was originally the outer limit of the retina, but 
becomes afterwards co-terminous with the membrana limitam 
externa, owing to the piercing through it of the external limbs of the 
rods. This system, which is known by the name of the radial 
Mullerian fibres (shown on the left-hand side of Fig. 41), has no 
connection with (2) the spongioblasts and neurospongium, which 
form a framework of neuroglia, in which the terminations of the 



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THE EVIDENCE OF THE ORGANS OF VISION 97 

optic ganglion and of the retinal ganglion ramify to form the mole- 
cular layers. 

It is evident from Fig. 41 that the retina of Ammoccetes and 
Petromyzon differs in a striking manner from the typical vertebrate 
retina. The epithelial part (C) remains the same — viz. the visual 
rods, the external limiting membrane, and the external nuclear 
layer; but the cerebral part, the retinal ganglion (A and B), is 
remarkably different. It is true, it consists in the main of the 
small-celled mass known as the inner nuclear layer, and of the 
reticulated tissue or * neuropil ' known as the inner molecular layer, 
just as in all other compound retinal eyes; but neither the ganglion 
cell-layer nor the optic fibre-layer is clearly defined as separate from 
this molecular layer ; on the contrary, it is matter of dispute as to 
what cells represent the ganglionic layer of higher vertebrates, and 
the optic fibres do not form a distinct innermost layer, but pass into 
the inner molecular layer at its junction with the inner nuclear 
layer. A comparison of this innermost part of the retina (A, Fig. 
41), with the corresponding part in Berger's picture of Musca (n.l.o.g., 
Fig. 38), shows a most striking similarity between the two. In both 
cases the fibres of the optic nerve (O.n., Fig. 38) which cross at their 
entrance pass into the ' neuropil ' of this part of the retinal ganglion, 
and are connected probably (though that is not proved in either 
case) with the cells of the ganglionic layer. In both cases we find 
two well-marked parallel rows of cells in this part of the retina, of 
which one, the innermost, is composed in Ammocoetes of large 
ganglion-cells, and the other mainly of smaller, deeper staining cells 
apparently supporting in function. Similarly, also, in Branchipus, as 
I conclude from my own observations as well as from those of Berger 
and Claus, the ganglionic layer is composed partly of true ganglion- 
cells and partly of supporting cells arranged in a distinct layer. This 
part, then, of the retina of Ammoccetes is remarkably like that of a 
typical arthropod retina, and forms that part of the retinal ganglion 
which may be called the ganglion of the optic nerve. 

Next comes the ganglion of the retina (B, Fig. 41) (Parker's first 
optic ganglion), the cells of which form the small bipolar granule- 
cells of the inner nuclear layer; granule-cells arranged in rows just 
as they are shown in Claus' picture of the same layer in the retina 
of Branchipus (Fig. 40), just as they are found in the cortical layers 
of the optic ganglion of the pineal eye (ganglion habenulm), in the 

11 



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98 THE ORIGIN OF VERTEBRATES 

optic lobes and other parts of the Ammocoetes brain, or in the cortical 
layers of the optic ganglia of all arthropods. 

Between this small-celled nuclear layer (4, Fig. 41) and the layer 
of nuclei of the visual rod cells (7, Fig. 41) (the external nuclear 
layer), we find in the eye of Ammocoetes and Petromyzon two well- 
marked rows of cells of a most striking character — viz. the two 
remarkably regular rows of large epithelial-like cells with large 
conspicuous nuclei, which give the appearance of two opposing rows 
of limiting epithelium (5, Fig. 41), already mentioned in connection 
with the researches of Langerhans and W. Miiller. Here, then, is a 
striking peculiarity of the retina of the lamprey, and according to 
Miiller the obliteration of these two layers can be traced as we pass 
upwards in the vertebrate kingdom. Among fishes, they are especially 
well seen in the perch ; in the higher vertebrates the whole layer is 
only a rudiment represented, he thinks, by the simple layer of round 
cells which lies close against the inner surface of the layer of 
terminal fibres (Nervenansatze), and is especially evident in birds 
and reptiles. In man and the higher mammals they are probably 
represented by the horizontal cells of the outer part of the inner 
nuclear layer. 

Seeing, then, that they are most evident in Ammocoetes, and 
become less and less marked in the higher vertebrates, it is clear 
that their origin cannot be sought among the animals higher in the 
scale than Ammocoetes, but must, therefore, be searched for in the 
opposite direction. 

Miiller describes them as forming a very conspicuous landmark in 
the embryology of the retina, dividing it distinctly into two parts, an 
outer thinner, and an inner somewhat thicker part, the zone formed 
by them standing out conspicuously on account of the size and regu- 
larity of the cells and their lighter appearance when stained. Thus 
in his description of the retina of an Ammocoetes 95 mm. in length, 
he says, "The layer of pale tangentially elongated cells formed a 
double layer and produced the appearance of a pale, very charac- 
teristic zone between the outer and inner parts of the retina." 

Let us now turn to the retina of the crustacean and see whether 
there is any evidence there that the retina is divisible into an outer 
and inner part, separated by a zone of characteristically pale staining 
cells with conspicuous nuclei. The most elaborate description of 
the development of the retina of Astacus is given by Eeichenbach, 



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THE EVIDENCE OF THE ORGANS OF VISION 99 

according to whom the earliest sign of the formation of the retina is an 
ectodermic involution ( Augen-einstiilpung), which soon closes, so that 
the retinal area appears as a thickening. In close contiguity to this 
thickening, the thickening of the optic ganglion arises, so that that 
part of the optic ganglion which will form the retinal ganglion fuses 
with the thickened optic plate and forms a single mass of tissue. 
Later on a fold (Augen-falte) appears in this mass of tissue, in conse- 
quence of which it becomes divided into two parts. The lining walls 
of this fold form a double row of cells, the nuclei of which are most 
conspicuous because they are larger and lighter in colour than the 
surrounding nuclei, so that by this fold the retina is divided into an 
outer and an inner wall, the line of demarcation being conspicuous by 
reason of these two rows of large, lightly-staining nuclei 

Beichenbach is unable to say that this secondary fold is coincident 
with the primary involution, and that therefore the junction between 
the two rows of large pale nuclei is the line of junction between the 
retinal ganglion and the retina proper, because all sign of the primary 
involution is lost before the secondary fold appears. 

Parker compares the appearances in the lobster with Beichenbach's 
description in the crayfish, and says that he finds only a thicken- 
ing, no primary involution; at the same time he expressly states 
that in the very early stages his material was deficient, and that he 
had not grounds sufficient to warrant the statement that no involution 
occurs. He also finds that in the lobster the ganglionic tissue which 
arises by proliferation is divided into an outer and inner part ; the 
separation is effected by a band of large, lightly-staining nuclei, which, 
in position and structure, resemble the band figured by Beichenbach. 
According to Parker, then, the line of separation indicated in the 
development by Beichenbach's outer and inner walls is not the line 
of junction between the retina and the retinal ganglion, as Beichen- 
bach was inclined to think, but rather a separation of two rows of 
large ganglion-cells belonging to the retinal ganglion. 

The similarity between these conspicuous layers of lightly- 
staining cells in Ammoccetes and in crustaceans is remarkably close, 
and in both cases observers have found the same difficulty in inter- 
preting their meaning. In each case one group of observers looks 
upon them as ganglion-cells, the other as supporting structures. 
Thus in the lamprey, Muller considers them to belong to the support- 
ing elements, while Langerhans and Kohl describe them as a double 



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IOO 



THE ORIGIN OF VERTEBRATES 







layer of ganglion-cells. In the crustacean, Berger in Squilla, Gren- 

acber in Mysis, and Parker in Astacus, look upon them as supporting 

elements, while Viallanes in 
Palinurus considers them to be 
true ganglionic cells. 

Whatever the final interpre- 
tation of these cells may prove 
to be, we may, it seems to me, 
represent an ideal compound 
retina of the crustacean type by 
combining the investigations of 
Berger, Claus, Eeichenbach, and 
Parker in the following figure. 

The comparison of this figure 
(Fig. 42) with that of the Pe- 
tromyzon retina (Fig. 41) shows 
how great is the similarity of 
the latter with the arthropod 
type, and how the very points 
in which it deviates from the 
recognized vertebrate type are 
explainable by comparison with 
that of the arthropod. The 
most striking difference between 
the retinas in the two figures is 
that the layer of terminal nerve 
fibres (5, Fig. 42), which, after 
all, are only the elongated termi- 
nations of the retinal cells be- 
longing to Parker's neurones of 
the first order, is very much 
longer than in Petromyzon or in 
any vertebrate, for the external 
molecular layer (6, Fig. 41) 
(Muller's layer of Nervenan- 

satze) is very short and inconspicuous (in Fig. 41 it is drawn too 

thick). 

Turning from the retina to the fibres of the optic nerve we again 

find a remarkable resemblance, for in Ammoccetes, as pointed out by 




Fig. 42.— Ideal Diagram op the Layers 
in a cru6tacean eye. 

The retina is divided into an epithelial 
part, C (the layer of retinular cells and 
rhabdomes), and a neurodermal or cere- 
bral part, which is formed of, A % the 
ganglion of the optic nerve, and, J3, the 
ganglion of the retina. 1, optic nerve 
fibres which cross at their entrance into 
the retina ; 2, int. molecular layer with 
its two rows of cells ; 8, int. nuclear 
layer; 4, Reichenbach's double row of 
large lightly-staining cells; 5, layer of 
terminal retinal fibres; 6, ext. nuclear 
layer ; 7, ext. limiting membrane ; 8, 
layer of crystalline cones ; 9, cornea. 



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THE EVIDENCE OF THE ORGANS OF VISION IOI 

Laugerhans and carefully figured by Kohl, a crossing of the fibres of 
the optic nerve occurs as the nerve leaves the retina, just as is so uni- 
versally the case in all compound retinas. To this crossing Kohl has 
given the name chiasma nervi optici, in distinction to the cerebral 
chiasma, which he calls chiasma nervorum opticorum. Further, we 
find that even this latter chiasma is well represented in the arthro- 
pod brain ; thus Bellonci in Sphseroma, Berger, Dietl, and Krieger in 
Astacus, all describe a true optic chiasma, the only difference in 
opinion being, whether the crossing of the optic nerves is complete or 
not. Especially instructive are Bellonci's figures and description. 
He describes the brain of Sphaeroma as composed of three segments 
—a superior segment, the cerebrum proper, a middle segment, 
and an inferior segment; the optic fibres, as is seen in Fig. 39, 
after crossing, pass direct into the middle segment, in the ganglia of 
which they terminate. From this segment also arises the nerve to 
the first antenna of that side — i.e. the olfactory nerve. The optic 
part, then, of this middle segment is clearly the brain portion of the 
optic ganglionic apparatus, and may be called the optic lobes, in 
contradistinction to the peripheral part, which is usually called the 
optic ganglion; and is composed of two ganglia, Op. g. I. and Op. g. II., 
as already mentioned. These optic lobes are therefore homologous 
with the optic lobes of the vertebrate brain. 

The resemblance throughout is so striking as to force one to the 
conclusion that the retina of the vertebrate eye is a compound retina, 
composed of a retina and retinal ganglion of the type found in arthro- 
pods. From this it follows that the development of the vertebrate 
retina ought to show the formation of (1) an optic plate formed 
from the peripheral epidermis and not from the brain ; (2) a part of 
the brain closely attached to this optic plate forming the retinal 
ganglion, which remains at the surface when the rest of the optic 
ganglion withdraws; (3) an optic nerve formed in consequence of 
this withdrawal, as the connection between the retinal and cerebral 
parts of the optic ganglion. 

This appears to me exactly what the developmental process does 
show according to Gotte's investigations. He asserts that the retina 
arises from an optic plate, being the optical portion of his * Sinnes- 
platte/ At an early stage this is separated by a furrow (Furche) 
from the general mass of epidermal cells which ultimately form the 
brain. This separation then vanishes, and the retina and brain-mass 



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102 THE ORIGIN OF VERTEBRATES 

become inextricably united into a mass of cells, which are still 
situated at the surface. By the closure of the cephalic plate and the 
withdrawal of the braiu away from the surface, a retinal mass of cells 
is left at the surface connected with the tubular central nervous 
system by the hollow optic diverticulum or primary optic vesicle. 
If we regard only the retinal and nervous elements, and for the 
moment pay no attention to the existence of the tube, Gotte's obser- 
vation that the true retina has been formed from the optic plate 
(Sinnes-platte) to which the retinal portion of the brain (retinal 
ganglion) has become firmly fixed, and that then the optic nerve has 
been formed by the withdrawal of the rest of the brain (optic lobes), 
is word for word applicable to the description of the development of 
the compound retina of the arthropod eye, as has been already stated. 

The Significance of the Optic Diverticula. 

The origin of the retina from an optic epidermal plate in verte- 
brates, as in all other animals, brings the cephalic eyes of all animals 
into the same category, and leaves the vertebrate eye no longer in an 
isolated and unnatural position. In one point the retina of the verte- 
brate eye differs from that of a compound retina of an invertebrate ; 
in the former, a striking supporting tissue exists, known as Muller's 
fibres, which is absent in the latter. This difference of structure is 
closely associated with another of the same character as in the central 
nervous system, viz. the apparent development of the nervous part from 
a tube. We see, in fact, that the retinal and nervous arrangements of 
the vertebrate eye are comparable with those of the arthropod eye, in 
precisely the same way and to the same extent as the nervous matter 
of the brain of the vertebrate is comparable with the brain of the 
arthropod. In both cases the nervous matter is, in structure, position, 
and function, absolutely homologous ; in both cases there is found in 
the vertebrate something extra which is not found in the invertebrate 
— viz. a hollow tube, the walls of which, in the case of the braiu, are 
utilized as supporting tissues for the nerve structures. The explana- 
tion of this difference in the case of the brain is the fundamental 
idea of my whole theory, namely, that the hollow tube is in reality 
the cephalic stomach of the invertebrate, around which the nervous 
brain -matter was originally grouped in precisely the same manner as 
in the invertebrate. What, then, are the optic diverticula ? 



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THE EVIDENCE OF THE ORGANS OF VISION 103 

" The formation of the eye," as taught by Balfour, " commences 
with the appearance of a pair of hollow outgrowths from the anterior 
cerebral vesicle. These outgrowths, known as the optic vesicles, at 
first open freely into the cavity of the anterior cerebral vesicle. 
From this they soon, however, become partially constricted, and 
form vesicles united to the base of the brain by comparatively 
narrow, hollow stalks, the rudiments of the optic nerves." 

" After the establishment of the optic nerves, there takes place 
(1) the formation of the lens, and (2) the formation of the optic cup 
from the walls of the primary optic vesicle." 

He then goes on to explain how the formation of the lens forms 
the optic cup with its double walls from the primary optic vesicle, 
and says — 

" Of its double walls, the inner, or anterior, is formed from the 
front portion, the outer, or posterior, from the hind portion of the 
wall of the primary optic vesicle. The inner, or anterior, which very 
speedily becomes thicker than the other, is converted into the retina ; 
in the outer, or posterior, which remains thin, pigment is eventually 
deposited, and it ultimately becomes the tesselated pigment-layer of 
the choroid." 

The difficulties in connection with this view of the origin of the 
eye are exceedingly great, so great as to have caused Balfour to 
discuss seriously Lankester's suggestion that the eye must have been 
at one time within the brain, and that the ancestor of the vertebrate 
was therefore a transparent animal, so that light might get to the eye 
through the outer covering and the brain-mass ; a suggestion, the 
unsatisfactory nature of which Balfour himself confessed. Is there 
really evidence of any part of either retina or optic nerve being 
formed from the epithelial lining of the tube ? 

This tube is formed as a direct continuation of the tube of the 
central nervous system, and we can therefore apply to it the same 
arguments as have been used in the discussion of the meaning of the 
latter tube. Now, the striking point in the latter case is the fact 
that the lining membrane of the central canal is in so many parts 
absolutely free from nervous matter, and so shows, as in the so-called 
choroid plexuses, its simple, non-nervous epithelial structure. This 
also we find in the optic diverticulum. Where there is no evidence 
of any invasion of the tube by nervous elements, there it retains its 
simple non-nervous character of a tube composed of a single layer of 



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104 THE ORIGIN OF VERTEBRATES 

epithelial cells — viz. in that part of the tube which, as Balfour says, 
remains thin, in which pigment is eventually deposited, and which 
ultimately becomes the tesselated pigment-layer of the choroid. 
Nobody has ever suggested that this pigment-layer is nervous matter, 
or ever was, or ever will be, nervous matter ; it is in precisely the 
same category as the membranous roof of the brain in Ammoccetes, 
which never was, and never will be, nervous matter. Yet, according 
to the old embryology both in the case of the eye and the brain, the 
pigment-layer and the so-called choroid plexuses are a part of the 
tubular nervous system. 

Turning now to the optic nerve, Balfour describes it as derived 
from the hollow stalk of the optic vesicle. He says — 

" At first the optic nerve is equally continuous with both walls 
of the optic cup, as must of necessity be the case, since the interval 
which primarily exists between the two walls is continuous with the 
cavity of the stalk. When the cavity within the optic nerve 
vanishes, and the fibres of the optic nerve appear, all connection is 
ruptured between the outer wall of the optic cup and the optic 
nerve, and the optic nerve simply perforates the outer wall, and 
becomes continuous with the inner one." 

In this description Balfour, because he derived the optic nerve 
fibres from the epithelial wall of the optic stalk, of necessity supposed 
that such fibres originally supplied both the outer and inner walls of 
the optic cup and, therefore, seeing that when the fibres of the optic 
nerve appear they do not supply the outer wall, he supposes that 
their original connection with the outer wall is ruptured, because a 
discontinuity of the epithelial lining takes place coincidently with 
the appearance of the optic nerve-fibres, and, according to him, the 
optic nerve simply perforates the outer wall and becomes continuous 
with the inner one. This last statement is very difficult to under- 
stand. I presume he meant that some of the fibres of the optic nerve 
supplied from the beginning the inner wall of the optic cup, but 
that others which originally supplied the outer wall were first ruptured, 
then perforated the outer wall, and finally completed the supply to 
the inner wall or retina. 

This statement of Balfour's is the necessary consequence of his 
belief, that the epithelial cells of the optic stalk gave rise to the 
fibres of the optic nerve. If, instead of this, we follow Kolliker and 
His, who state that the optic nerve-fibres are formed outside the 



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THE EVIDENCE OF THE ORGANS OF VISION 105 



epithelial walls of the optic stalk, and that the cells of the latter 
form supporting structures for the nerve-fibres, then the position of 
the optic nerve becomes perfectly simple and satisfactory without 
any rupturing of its connection with the outer wall and subsequent 
perforation, for the optic nerve-fibres from their very first appearance 
pass directly to supply the retina — i.e. the inner wall of the optic 
cup and nothing else. 

They pass, as is well known, without any perforation by way of 
the choroidal slit to the inner surface of the inner wall (retina) of 
the optic cup; then, when the choroidal 
slit becomes closed by the expansion \ 

of the optic cup, the optic nerve 
naturally becomes situated in the centre 
of the base of the cup and spreads over 
its inner surface as that surface expands. 

A section across the optic cup at an 
early stage at the junction of the optic 
stalk and optic cup would be repre- 
sented by the upper diagram in Fig. 
43 ; at a later stage, when the choroidal 
slit is closed, by the lower diagram. 

The evident truth of this manner 
of looking at the origin of the optic 
nerve is demonstrated by the appear- 
ance of the optic nerve in Ammo- 
cijetes and Petromyzon. In the latter, Fio. 43 
although the development is complete, 
and the eye, and consequently also the 
optic nerve-fibres, are fully functional, 
there is still present in the axial core 
of the nerve a row of epithelial cells 
(Axenstrang) which are altered so as 
to form supporting structures, in the 
same way as a row of epithelial cells in the retina is altered to form 
the system of supporting cells known by the name of the Mlillerian 
fibres. 

The origiu of this axial core of cells is perfectly clear, as has been 
pointed out by W. Miiller. He says — 

" The development of the optic nerve shows peculiarities in 




On 

Diagram of the Rela- 
tion of the Optic Nkhve to 
the Optic Cup. 

The upper diagram represents a 
stage before the formation of the 
choroidal slit, the lower one the 
stage of closure of the choroidal 
slit. R., retina; O.n., optic 
nerve ; p., pigment epithelium. 



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106 THE ORIGIN OF VERTEBRATES 

Petromyzon of such a character as to make this animal one of the 
most valuable objects for deciding the various controversial questions 
connected with the genesis of its elements. The lumen of the stalk 
of the primary optic vesicle is obliterated quite early by a prolife- 
ration of its lining epithelium. Also the original continuity of this 
epithelium with that of the pigment-layer is at an early period 
interrupted at the point of attachment of the optic stalk. This 
interruption occurs at the time when the fibres of the optic nerve 
first become visible." 

Further on he says — 

" The epithelium of the optic stalk develops entirely into sup- 
porting cells, which in Petromyzon fill up the original lumen and so 
form an axial core (Axenstrang) to the nerve-fibres which are formed 
entirely outside them ; the projections of these supporting cells are 
directed towards the periphery, and so separate the bundles of the 
optic nerve-fibres. The mesodermal coat of the optic stalk takes no 
part in this separation ; it simply forms the connective tissue sheath 
of the optic nerve. The development of the optic nerve in the 
higher vertebrates also obeys the same law, as I am bound to conclude 
from my own observations." 

The evidence, then, of Ammocoetes is very conclusive. Originally 
a tube composed of a single layer of epithelial cells became expanded 
at the anterior end to form a bulb. On the outside of this tube or 
stalk the fibres of the optic nerve make their appearance, arising from 
the ganglion-cell layer of the retina, and, passing over the surface of 
the enithelial tube at the choroidal fissure, proceed to the brain by 
way of the optic chiasma. Owing to the large number of fibres, their 
crossing at the junction of the stalk with the bulb, and the narrow- 
ness at this neck, the obliteration of the lumen of the tube which 
takes place in the stalk is carried out to a still greater extent at this 
narrow part. The result of this is that all continuity of the cell- 
layers of the original tube of the optic stalk with those of both the 
inner and outer walls of the bulb is interrupted, and all that remains 
in this spot of the original continuous line of cells which connected the 
tube of the stalk with that of the bulb are possibly some of the groups 
cells which are found scattered among the fibres of the optic nerve 
their entrance into the retina. Such separation of the originally 

f inuous elements of the epithelial wall of the optic stalk, which 
^arent only at this neck of the nerve in Petromyzon, takes place 



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THE EVIDENCE OF THE ORGANS OF VISION 107 

along the whole of the optic nerve in the higher vertebrates, so that 
no continuous axial core of cells exist, but only scattered supporting 
cells. 

If further proof in support of this view be wanted, it is given by 
the evidence of physiology, which shows that the fibres of the optic 
nerve are not different from other nerve-fibres of the central nervous 
system, but that they degenerate when separated from their nerve- 
cell, and that the nerve-cell of which the optic nerve-fibre is a 
process is the large ganglion-cell of the ganglionic layer of the retina. 
The origin of the ganglionic layer of the retina cannot therefore be 
separated from that of the optic nerve-fibres. If the one is outside 
the epithelial tube, so is the other, and what holds true of the gan- 
glionic layer must hold good of the rest of the retinal ganglion and, 
from all that has been said, of the retina itself. We therefore come 
to the conclusion that the evidence is distinctly in favour of the 
view, that the retina and optic nerve in the true sense are structures 
which originally were outside a non-nervous tube, but, just like the 
central nervous system as a whole, have amalgamated so closely with 
the elements of this tube as to utilize them for supporting structures. 
One part of this non-nervous tube, its dorsal wall, like the corre- 
sponding part of the brain-tube, still retains its original character, 
and by the deposition of pigment has been pressed into the service 
of the eye to form the pigmented epithelial layer. 

We can, however, go further than this, for we know definitely in 
the ca9e of the retina what the fate of the epithelial cells lining 
this tube has been. They have become the system of supporting 
structures known as Miillerian fibres. 

The epithelial layer of the primary optic vesicle can be traced into 
direct continuity with the lining epithelium of the brain cavity, as 
a single layer of epithelial cells in the core of the optic nerve, form- 
ing the optic stalk, which, in consequence of close contact, becomes 
the well-known axial layer of supporting cells. This epithelial layer 
of the optic stalk then expands to form the optic bulb, the outer or 
dorsal wall of which still remains as a single layer of epithelium 
and becomes the layer of pigment epithelium. This layer of 
epithelium becomes doubled on itself by the approximation of the 
inner or ventral wall of the optic cup to the outer or dorsal wall in 
consequence of the presence of the lens, and still remaining a single 
layer, forms the pars ciliaris retince ; then suddenly, at the ora 



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io8 



THE ORIGIN OF VERTEBRATES 



scrrata, the single epithelial layer vanishes, and the layers of the 
retina take its place. It has long been known, however, that even 
throughout the retina this single epithelial layer still continues, being 
known as the fibres of Muller. This is how the fact is described 
in the last edition of Foster's "Text-book of Physiology," p. 1308— 

" Stretching radially from the inner to the outer limiting mem- 
brane in all regions of the retina are certain peculiar- shaped bodies 
known as the radial fibres of Muller. Each fibre is the outcome of 
the changes undergone by what was at first a simple columnar 

epithelial cell. The changes 
are, in the main, that the 
columnar form is elongated 
into that of a more or less 
prismatic fibre, the edges of 
which become variously 
branched, and that while the 
nucleus is retained the cell 
substance becomes converted 
into neuro-keratin. And, in- 
deed, at the ora semata the 
fibres of Muller may be seen 
suddenly to lose their peculiar 
features and to pass into the 
ordinary columnar cells which 
form the 2>ars ciliaris retime." 
It is then absolutely clear 
that the essential parts of the 
eye may be considered as 
composed of two parts — 

1. A tube or diverticulum 
from the tube of the central nervous system, composed throughout 
of a single layer of epithelium, which forms the supporting axial 
cells in the optic nerve, the pigment epithelium and the Miillerian 
fibres of the retina. Such a tube would be represented by the 
accompanying Fig. 44, and the left side of Fig. 41. 

2. The retina proper with the retinal ganglion and the optic 
nerve-fibres as already described. In this part supporting elements 
are found, just as in any other compound retina, of the nature of 
neuroglia, which are independent of the Miillerian fibres. 




Fig. 44. — Diagram representing the 
slngle-layered epithelial tube of 
the Vertebrate Eye after removal of 
the Nervous and Retinal Elements. 

O.n., axial core of cells in optic nerve; p., 
pigment epithelium; p.c.r., pars ciliaris 
retime ; m.f., Miillerian fibres; l. t lens. 



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THE EVIDENCE OF THE ORGANS OF VISION 109 

Of these two parts we have already seen that the second is to 
all intents and purposes a compound retina of a crustacean eye, and 
seeing that the single-layered epithelial tube is continuous with the 
single-layered epithelial tube of the central nervous system — i.e. with 
the cephalic part of the gut of the arthropod ancestor — it follows with 
certainty that the ancestor of the vertebrates must have possessed 
two anterior diverticula of the gut, with the wall of which, near the 
anterior extremity, the compound retina has amalgamated on either 
side, just as the infra-cesophageal ganglia have amalgamated with 
the ventral wall of the main gut-tube. In this way, and in this way 
alone, does the interpretation of the structure of the vertebrate lateral 
eye harmonize in the most perfect manner with the rest of the con- 
clusions already arrived at. 

The question therefore arises : — Have we any grounds for believing 
that the ancient forms of primitive crustaceans and primitive arachnids, 
which were so abundant in the time when the Cephalaspids appeared, 
possessed two anterior diverticula of the stomach, such as the con- 
sideration of the vertebrate eye strongly indicates must have been 
the case ? 

The beautiful pictures of Blanchard, and his description, show 
how, on the arachnid side, paired diverticula of the stomach are 
nearly universal in the group. Thus, although they are not present 
in the scorpions, still, in the Thelyphonidae, Phrynidae, Solpugidae, 
Mygalidae, the most marked characteristic of the stomach-region is 
the presence of four pairs of ccecal diverticula, which spread laterally 
over the prosomatic region. In the spiders the number of such 
diverticula increases, and the whole prosomatic region becomes filled 
up with these tubes. Blanchard considers that they form nutrient 
tubes for the direct nutrition of the organs in the prosoma, especially 
the important brain-region of the central nervous system. He points 
out that these animals are blood-suckers, and that, therefore, their 
food is already in a suitable form for purposes of nutrition when it 
is taken in by them, so that, as it were, the anterior part of the gut 
is transformed into a series of vessels or diverticula conveying blood 
directly to the important organs in the prosoma, by means of which 
they obtain nourishment in addition to their own blood-supply. 

The universality of such diverticula among the arachnids makes 
it highly probable that their progenitors did possess an alimentary 
canal with one or more pairs of anterior diverticula. In the 



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no 



THE ORIGIN OF VERTEBRATES 



vertebrate, however, the paired diverticula are associated with a 
compound retina, a combination which does not occur among living 
arachnids ; we must, therefore, examine the crustacean group for the 
desired combination, and naturally the most likely group to examine 
is the Phyllopoda, especially such primitive forms as Branchipus and 
Artemia, for it is universally acknowledged that these forms are 




— rt.gl 



Fig. 45.— Section through one of the two Anterior Diverticula op the Gut 
in Artemia and the Retinal Ganglion. 

The section is through the extreme anterior end of the diverticulum, thus cutting 
through many of the columnar cells at right angles to their axis. Al. f gut 
diverticulum ; rt. gl., retinal ganglion. 

the nearest living representatives of the trilobites. If, therefore, it 
be found that the retina and optic nerve in Artemia is in specially 
close connection with an anterior diverticulum of the gut on each 
side, then it is almost certain that such a combination existed also 
in the trilobites. 

My friend Mr. W. B. Hardy has especially investigated the 
nervous system of Artemia. In the course of his work he cut serial 



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le 



THE EVIDENCE OF THE ORGANS OF VISION III 

sections through the whole animal, and, as mentioned in my paper 
in the Journal of Anatomy and Physiology, he discovered that the 
retinal ganglion of each c.e On 

lateral eye is so closely 
attached to the end of the 
corresponding diverticu- 
lum of the gut that the 
lining cells of the ventral 
part of the diverticulum 
form a lining to the reti- 
nal ganglion (Fig. 45). In 
this animal there are only 
two gut-diverticula, which 
are situated most ante- 
riorly. I have plotted 
out this series of sections 
by means of a camera 
lucida, with the result 
that the retina appears as 
a bulging attached ventro- 
lateral^ to the extremity of each gut-diverticulum, as is shown in 




Al 

Fig. 46. — The Brain, Eyes, and Anterior 
Termination of the Alimentary Canal op 
Artemia, viewed from the Dorsal Aspect. 

Br., brain; I.e., lateral eyes; c.e., median eyes; AL, 
alimentary canal. 





A B 

Fig. 47.— A, The Formation of the Retina of the Eye of Ammoccetes (after 
Scott) ; B, The Formation of the Retina of the Eye of Ammoccetes, on 
my theory. 
R., retina; *., lens; O.n., optic nerve fibres; AL, cephalic end of invertebrate ali- 
mentary canal; I'., cavity of ventricles of brain; Al.d., anterior diverticulum 
of alimentary canal ; op.d., optic diverticulum. 

Fig. 46. It is instructive to compare with this figure Scott's picture 
of the developing eye in Ammocootes, where he figures the retina as 



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112 THE ORIGIN OF VERTEBRATES 

a bulging attached ventrally to the extremity of the narrow tube of 
the optic diverticulum. In Fig. 47, A, I reproduce this figure of 
Scott, and by the side of it, Fig. 47, B, I have represented the origin 
of the vertebrate eye as I believe it to have occurred. 

We see, then, this very striking fact, that in the most primitive 
of the Crustacea, not only are there two anterior diverticula of the 
gut, but also the retinal ganglion of the lateral eye is in specially 
close connection with the end of the diverticulum on each side. In 
fact, we find in the nearest living representative of the trilobites a 
retina and retinal ganglion and optic nerve, closely resembling that 
of the vertebrate, in close connection with an epithelial tube which 
has nothing to do with the organ of sight, but is one of a pair of 
anterior gut-diverticula. It is impossible to obtain more decisive 
evidence that the trilobites possessed a pair of gut-diverticula sur- 
rounded to a greater or less extent by the retina and optic nerve of 
each lateral eye. 

Such anterior diverticula are commonly found in the lower 
Crustacea ; they are usually known by the name of liver-diverticula, 
but as they take no part in digestion, and, on the contrary, represent 
that part of the gut which is most active in absorption, the term 
liver is not appropriate, and it is therefore better to call them simply 
the pair of anterior diverticula. Our knowledge of their function in 
Daphnia is given in a paper by Hardy and M'Dougall, which does 
not appear to be widely known. Hardy succeeded in feeding Daphnia 
with yolk of egg in which carmine grains were mixed, and was able 
in the living animal to watch the whole process of deglutition, 
digestion, and absorption. The food, which is made into a bolus, is 
moved down to the middle region of the gut, and there digestion 
takes place. Then by an antiperistaltic movement the more fluid 
products of the digestion-process are sent right forward into the two 
anterior diverticula, where the single layer of columnar cells lining 
these diverticula absorbs these products, the cells becoming thickly 
studded with fat-drops after a feed of yolk of egg. The carmine 
particles, which were driven forward with the proteid- and fat- 
particles, are not absorbed, but are at intervals driven back by con- 
tractions of the anterior diverticula to the middle region of the gut. 

These observations prove most clearly that the anterior diver- 
ticula have a special nutrient function, being the main channels by 
which new nutrient material is brought into the body, and, as 



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THE EVIDENCE OF THE ORGANS OF VISION 113 

pointed out by the authors, it is a remarkable exception in the 
animal kingdom that absorption should occur in that portion of the 
gut which is anterior to the part in which digestion occurs. In all 
these animals the two anterior diverticula extend forwards over the 
brain, and, as we have seen in Artemia, the anterior extremity 
of each one is so intimately related to a part of the brain— viz. 
the retinal ganglion — as to form a lining membrane to that mass 
of nerve-cells. It follows, therefore, that the nutrient fluid absorbed 
by the cells of this part of the gut-diverticulum must be primarily 
for the service of the retinal ganglion. In fact, the relations of 
this anterior portion of the gut to the brain as a whole suggest 
strongly that the marked absorptive function of this anterior 
portion of the gut exists in order to supply nutrient material 
in the first place to the most vital, most important organ in the 
animal— the brain and its sense-organs. This conclusion is borne 
out by the fact that in these lower crustaceans the circulation of 
blood is of a very inefficient character, so that the tissues are mainly 
dependent for their nutrition on the fluid immediately surrounding 
them. It stands to reason that the establishment of the anterior 
portion of the gut as a nutrient tube to the brain would necessitate 
a closer and closer application of the brain to that tube, so that the 
process of amalgamation of the brain with the single layer of columnar 
epithelial cells which constitutes the wall of the gut (which we see 
in its initial stage in the retinal ganglion of Artemia), would tend 
rapidly to increase as more and more demands were made upon the 
brain, until at last both the supra- and infra-oesophageal ganglia, as 
well as the retinal ganglia and optic nerves, were in such close 
intimate connection with the ventral wall of the anterior portion of 
the gut and its diverticula as to form a brain and retina closely 
resembling that of Ammoccetes. 

Such an origin for the lateral eyes of the vertebrate explains in a 
simple and satisfactory manner why the vertebrate retina is a com- 
pound retina, and why both retina and optic nerve have an apparent 
tubular development. 

At the same time one discrepancy still exists which requires 
consideration — viz. in no arthropod eye possessing a compound 
Tetina is the retina inverted. All the known cases of inversion 
among arthropods occur in eyes, the retina of which is simple, and 
are all natural consequences of the process of invagination by which 

I 



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114 THE ORIGIN OF VERTEBRATES 

the retina is formed. On the other hand, eyes with an inverted 
compound retina are not entirely unknown among invertebrates, for 
the eyes of Pecten and of Spondylus possess a retina which is 
inverted after the vertebrate fashion and still may be spoken of as 
compound rather than simple. It is clear that an invagination, the 
effect of which is an inversion of the retinal layer, would lead to 
the same result, whether the retinal optic nerves were short or long, 
whether, in fact, a retinal ganglion existed or not. Undoubtedly the 
presence of the retinal ganglion tends greatly to obscure any process 
of invagination, so that, as already mentioned, many observers, with 
Parker, consider the retina of the crustacean lateral eye to be 
formed by a thickening only, without any invagination, while 
Reichenbach says an obscure invagination does take place at a very 
early stage. So in the vertebrate eye most observers speak only of 
a thickening to form the retina, but Gotte's observation points to an 
invagination of the optic plate at an early stage. So also in the eye 
of Pecten, Korschelt and Heider consider that the thickening, by 
which the retina is formed according to Patten, in reality hides an 
invagination process by means of which, as Biitschli suggests, an 
optic vesicle is formed in the usual manner. The retina is 
formed from the anterior wall of this vesicle, and is therefore 
inverted. 

The origin of the inverted retina of the vertebrate eye does not 
seem to me to present any great difficulty, especially when one 
takes into consideration the fact that the retina is inverted in the 
arachnid group, only in the lateral eyes. The inversion is 
usually regarded as associated with the tubular formation of the 
vertebrate retina, and it is possible to suppose that the retina became 
inverted in consequence of the involvement of the eye with the gut- 
diverticulum. I do not myself think any such explanation is at all 
probable, because I cannot conceive such a process taking place with- 
out a temporary derangement — to say the least of it — of the power of 
vision, and as I do not believe that evolution was brought about by 
sudden, startling changes, but by gradual, orderly adaptations, and 
as I also believe in the paramount importance of the organs of 
vision for the evolution of all the higher types of the animal kingdom, 
I must believe that in the evolution from the Arthropod to the 
Cephalaspid, the lateral eyes remained throughout functional. I 
therefore, for my own part, would say that the inversion of the 



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THE EVIDENCE OF THE ORGANS OF VISION 115 

retina took place before the complete amalgamation with the gut- 
diverticulum, that, in fact, among the proto-crustacean, proto- 
arachnid forms there were some sufficiently arachnid to have an 
inverted retina, and at the same time sufficiently crustacean to 
possess a compound retina, and therefore a compound inverted 
retina after the vertebrate fashion existed in combination with the 
anterior gut-diverticula. Thus, when the eye and optic nerve sank 
into and amalgamated with the gut-divert iculum, neither the dioptric 
apparatus nor the nervous arrangements would suffer any alteration, 
and the animal throughout the whole process would possess organs 
of vision as good as before or after the period of transition. 

Further, not only the retina but also the dioptric apparatus of 
the vertebrate eye point to its origin from a type that combined 
the peculiarities of the arachnids and the crustaceans. In the 
former it is difficult to speak of a true lens, the function of a lens 
being undertaken by the cuticular surface of the cells of the corneagen 
(Mark's ' lentigcn '), while in the latter, in addition to the corneal 
covering, a true lens exists in the shape of the crystalline cones. 
Further, these crustacean lenses are true lenses in the vertebrate 
sense, in that they are formed by modified hypodermal cells, and 
not bulgings of the cuticle, as in the arachnid. We see, in fact, that 
in the compound crustacean eye an extra layer of hypodermal cells has 
become inserted between the cornea and the retina to form a lens. 
So also in the vertebrate eye the lens is formed by an extra layer of 
the epidermal cells between the cornea and the retina. The fact that 
the vertebrate eye possesses a single lens, though its retina is composed 
of a number of ommatidia, while the crustacean eye possesses a lens 
to each ommatidium, may well be a consequence of the inversion of 
the vertebrate retina. It is most probable, as Korschelt and Heider 
have pointed out, that the retina of the arachnid eyes is composed 
of a number of ommatidia, just as in the crustacean eyes and 
in the inverted eyes it is probable that the image is focussed on 
to the pigmented tapetal layer, and thence reflected on to the 
percipient visual rods. In such a method of vision a single lens is a 
necessity, and so it must also be if, as I suppose, eyes existed with 
an inverted compound retina. Owing to the crustacean affinities of 
such eyes, a lens would be formed and the retina would be compound : 
owing to the arachnid affinities, the retina would be inverted and 
the hypodermal cells which formed the lens would l>e massed 



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Il6 THE ORIGIN OF VERTEBRATES 

together to form a single lens, instead of being collected in groups of 
four to form a series of crystalline cones. 

To sum up : The study of the vertebrate eyes, both median and 
lateral, leads to most important conclusions as to the origin of the 
vertebrates, for it shows clearly that whereas, as pointed out in this 
and subsequent chapters, their ancestors possessed distinct arachnid 
characteristics, yet that they cannot have been specialized arachnids, 
such as our present-day forms, but rather they were of a primitive 
arachnid type, with distinct crustacean characteristics : animals 
that were both crustacean and arachnid, but not yet specialized in 
either direction: animals, in fact, of precisely the kind which 
swarmed in the seas at the time when the vertebrates first made their 
appearance. In the opinion of the present day, the ancestral forms 
of the Crustacea, which were directly derived from the Annelida, 
may be classed as an hypothetical group the Protostraca, the nearest 
approach to which is a primitive Phyllopod. 

" Starting from the Protostraca," say Korschelt and Heider, 
" according to the present condition of our knowledge, we may, as 
has been already remarked, assume three great series of development 
of the Arthropodan stock, by the side of which a number of smaller 
independent branches have been retained. One of these series leads 
through the hypothetical primitive Phyllopod to the Crustacea ; the 
second through the Palaeostraca (Trilobita, Gigantostraca, Xiphosura) 
to the Arachnida ; the third through forms resembling Peripatus to 
the Myriapoda and the Insecta. The Pantapoda and the Tardigrada 
must probably be regarded as smaller independent branches of the 
Arthropodan stock." 

To these " three great series of development of the Arthropodan 
stock " the evidence of Ammoccetes shows that a fourth must be added, 
which, starting also from the Protostraca, and closely connected with 
the second, palseostracan branch, leads through the Cephalaspidre to 
the great kingdom of the Vertebrata. Such a direct linking of the 
earliest vertebrates with the Annelida through the Protostraca is of 
the utmost importance, as will be shown later in the explanation of 
the origin of the vertebrate ccelom and urinary apparatus. 



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THE EVIDENCE OF THE ORGANS OF VISION 117 



Summary. 

The most important discovery of recent years which gives a direct clue to 
the ancestry of the vertebrates is undoubtedly the discovery that the pineal gland 
is all that remains of a pair of median eyes which must have been functional in 
the immediate ancestor of the vertebrate, seeing how perfect one of them 
still is in Ammocoetes. The vertebrate ancestor, then, possessed two pairs of 
eyes, one pair situated laterally, the other median. In striking confirmation of 
the origin of the vertebrate from Palfeostracans it is universally admitted that 
all the Eurypterids and such -like forms resembled Limulus in the possession of 
a pair of median eyes, as well as of a pair of lateral eyes. Moreover, the ancient 
mailed fishes the Ostracodermata, which are the earliest fishes known, are all said 
to show the presence of a pair of median eyes as well as of a pair of lateral eyes. 
This evidence directly suggests that the structure of both the median and 
lateral vertebrate eyes ought to be very similar to that of the median and lateral 
arthropod eyes. Such is, indeed, found to be the case. 

The retina of the simplest form of eye is formed from a group of the superficial 
epidermal cells, and the rods or rhabdites are formed from the cuticular covering 
of these cells ; the optic nerve passes from these cells to the deeper-lying brain. 
This kind of retina may be called a simple retina, and characterizes the eyes, 
both median and lateral, of the scorpion group. 

In other cases a portion of the optic ganglion remains at the surface, when 
the brain sinks inwards, in close contiguity to the epidermal sense-cells which 
form the retina ; a tract of fibres connects this optic ganglion with the under- 
lying brain, and is known as the optic nerve. Such a retina may be called 
a compound retina and characterizes the lateral eyes of both crustaceans and 
vertebrates. Also, owing to the method of formation of the retina by invagina- 
tion, the cuticular surface of the retinal sense-cells, from which the rods are 
formed, may be directed towards the source of light or away from it. In the 
first case the retina may be called upright, in the second inverted. 

Such inverted retinas are found in the vertebrate lateral eyes and in the 
lateral eyes of the arachnids, but not of the crustaceans. 

The evidence shows that all the invertebrate median eyes possess a simple 
upright retina, and in structure are remarkably like the right median or pineal 
eye of Ammocoetes; while the lateral eyes possess, as in the crustaceans, an 
upright compound retina, or, as in many of the arachnids, a simple inverted 
retina. The lateral eyes of the vertebrates alone possess a compound inverted 
retina. 

This retina, however, is extraordinarily similar in its structure to the 
compound crustacean retina, and these similarities are more accentuated in the 
retina of the lateral eye of Petromyzon than that of the higher vertebrates. 

The evidence afforded by the lateral eye of the vertebrate points unmistakably 
to the conclusion that the ancestor of the vertebrate possessed both crustacean 
and arachnid characters — belonged, therefore, to a group of animals which gave 
rise to both the crustacean and arachnid groups. This is precisely the position 
of the Palseostracan group, which is regarded as the ancestor of both the 
crustaceans and arachnids. 



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Il8 THE ORIGIN OF VERTEBRATES 

In two respects the retina of the lateral eyes of vertebrates differs from that 
of all arthropods, for it possesses a special supporting" structure, the Mullerian 
fibres, which do not exist in the latter, and it is developed in connection with 
a tube, the optic diverticulum, which is connected on each side with the main 
tube of the central nervous system. These two differences are in reality one 
and the same, for the Mullerian fibres are the altered lining cells of the optic 
diverticulum, and this tube has the same significance as the rest of the tube of 
the nervous system ; it is something which has nothing to do with the nervous 
portion of the retina but has become closely amalgamated with it. The explana- 
tion is, word for word, the same as for the tubular nervous system, and shows that 
the ancestor of the vertebrate possessed two anterior diverticula of its alimentary 
canal which were in close relationship to the optic ganglion and nerve of the 
lateral eye on each side. It is again a striking coincidence to find that 
Artemia, which with Branchipus represents a group of living crustaceans most 
nearly allied to the trilobites, does possess two anterior diverticula of the gut 
which are in extraordinarily close relationship with the optic ganglia of the 
retina of the lateral eyes on each side. 

The evidence of the optic apparatus of the vertebrate points most remarkably 
to the derivation of the Vertebrata from the Palseostraca. 



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CHAPTER III 
THE EVIDENCE OF THE SKELETON 

The bony and cartilaginous skeleton considered, not the notochord. — Nature of 
the earliest cartilaginous skeleton. — The mesosomatic skeleton of Ammo- 
coetes; its topographical arrangement, its structure, its origin in muco- 
cartilage. — The prosomatic skeleton of Ammocoetes; the trabeculse and 
parachordals, their structure, their origin in white fibrous tissue.— The 
mesosomatic skeleton of Limulus compared with that of Ammocoetes; 
similarity of position, of structure, of origin in muco-cartilage.— The 
prosomatic skeleton of Limulus ; the entosternite or plastron compared with 
the trabecule of Ammocoetes; similarity of position, of structure, of origin 
in fibrous tissue. — Summary. 

The explanation of the two optic diverticula given in the last chapter 
accounts in the same harmonious manner for every other part of the 
tube around which the central nervous system of the vertebrate has 
been grouped. The tube conforms in all respects to the simple epi- 
thelial tube which formed the alimentary canal of the ancient type of 
marine arthropods such as were dominant in the seas when the verte- 
brates first appeared. The whole evidence so far is so uniform and 
points so strongly in the direction of the origin of vertebrates from 
these ancient arthropods, as to make it an imperative duty to proceed 
further and to compare one by one the other parts of the central 
nervous system, together with their outgoing nerves in the two groups 
of animals. 

Before proceeding to do this, it is advisable first to consider 
the question of the origin of the vertebrate skeletal tissues, for this 
is the second of the great difficulties in the way of deriving verte- 
brates from arthropods, the one skeleton being an endo-skeleton 
composed of cartilage and bone, and the other an exo-skeleton com- 
posed of chitin. Here is a problem of a totally different kind to that 
we have just beeu considering, but of so fundamental a character that 
it must, if possible, be solved before passing on to the consideration 
of the cranial nerves and the organs they supply. 



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V 



120 THE ORIGIN OF VERTEBRATES 

Is there any evidence which makes it possible to conceive the 
method by which the vertebrate skeleton may have arisen from the 
skeletal tissues of an arthropod ? By the vertebrate skeleton I mean 
the bony and cartilaginous structures which form the backbone, the 
cranio-facial skeleton, the pectoral and pelvic girdles, and the bones 
of the limbs. I do not include the notochord in these skeletal tissues, 
because there is not the slightest evidence that the notochord played 
any part in the formation of these structures ; the notochordal tissue 
is something mi generis, and never gives rise to cartilage or bone. 
The notochord happens to lie in the middle line of the body and is 
very conspicuous in the lowest vertebrate ; with the development of 
the backbone the notochord becomes obliterated more and more, until 
at last it is visible in the higher vertebrates only in the embryo ; but 
that obliteration is the result of the encroachment of the growing 
bone-masses, not the cause of their growth. Although, then, the 
notochord may in a sense be spoken of as the original supporting axial 
rod of the vertebrate, it is so different to the rest of the endo-skeleton, 
has so little to do with it, that the consideration of its origin is a thing 
apart, and must be treated by itself without reference to the origin of 
the cartilaginous and bony skeleton. 

The Commencement of the Bony Skeleton in the Vertebrate. 

What is the teaching of the vertebrate ? What evidence is there 
as to the origin of the bony skeleton in the vertebrate phylum 
itself ? 

The axial bony skeleton of the higher Mammalia consists of two 
parts, (1) the vertebral column with its attached bony parts, and 
(2) the cranio-facial skeleton. Of these two parts, the bony tissue 
of the first arises in the embryo from cartilage, of the second partly 
from cartilage, partly from membrane. 

In strict accordance with their embryonic origin is their phyloge- 
netic origin: as we pass from the higher vertebrates to the lower 
these structures can be traced back to a cartilaginous and mem- 
branous condition, so that, as Parker has shown, the cranio-facial 
bony skeleton of the higher vertebrates can be derived directly from 
a non-bony cartilaginous skeleton, such as is seen in Petromyzon 
and the cartilaginous fishes. 

Balfour, in his " Comparative Embryology," states that the 



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THE EVIDENCE OF THE SKELETON 



121 



primitive cartilaginous cranium is always composed of the following 
parts : — 

1. A pair of cartilaginous plates on each side of the cephalic 
section of the notochord known as the parachordals (pa.ch., Fig. 49 ; 
iv., Fig. 48). These plates, together with the notochord (ch.) enclosed 
between them, form a floor for the hind and mid-brain. 




-~au 



Fig. 48.— Embryo Pig, two-thibds of an 
inch long (from Parkeb), Elements 
of Skull been from below. 

ch., notochord; iv., parachordals; au., 
auditory capsule ; py., pituitary body ; tr., 
trabecula; dr., trabecular cornu; pn., 
pre-nasal cartilage ; ppg., palato-pterygoid 
tract ; mn., mandibular arch ; th.h., first 
branchial arch ; VII.-XIL, cranial nerves. 



Fig. 49. — Head of Embryo Dog-fish 
(from Parker), Basal View of Cranium 

FROM ABOVE. 

ol. t olfactory sacs; aw., auditory capsule; 
py., pituitary body; pa.ch., parachordal 
cartilage; tr., trabecula; inf., infundi- 
bulum ; pt.s., pituitary space ; c, eye. 



2. A pair ot bars forming the floor for the fore-brain, known as 
the trabecule (tr). These bars are continued forward from the para- 
chordals. They meet posteriorly and embrace the front end of the 
notochord, and after separating for some distance bend in again in 
such a way as to enclose a space — the pituitary space (pt.s.). In 



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122 THE ORIGIN OF VERTEBRATES 

front of this space they remain in contact, and generally unite. They 
extend forward into the nasal region (pn.). 

3. The cartilaginous capsules of the sense organs. Of these the 
auditory (au.) and the olfactory capsules (ol.) unite more or less inti- 
mately with the cranial walls ; while the optic capsules, forming the 
usually cartilaginous sclerotics, remain distinct. 

The parachordals and notochord form together the basilar plate, 
which forms the floor for that section of the brain belonging to 
the primitive postoral part of the head, and its extent corresponds 
roughly to that of the basioccipital of the adult skull. 

The trabecule, so far as their mere anatomical relations are con- 
cerned, play the same part in forming the floor for the front cerebral 
vesicle as do the parachordals for the mid- and hind-brain. They 
differ, however, from the parachordals in one important feature, viz. 
that except at their hinder end they do not embrace the notochord. 
The notochord always terminates at the infundibulum, and the 
trabecule always enclose a pituitary space, in which lies the infun- 
dibulum (inf.) and the pituitary body (i>y.). 

In the majority of the lower forms the trabecuhe arise quite inde- 
pendently of the parachordals, though the two sets of elements soon 
unite. 

The trabecuhe are usually somewhat lyre-shaped, meeting in 
front and behind, and leaving a large pituitary space between their 
middle parts. Into this space the whole base of the fore-brain 
primitively projects, but the space itself gradually becomes nan*owed 
until it usually contains only the pituitary body. 

The trabecular floor of the brain does not long remain simple. 
Its sides grow vertically upwards, forming a lateral wall for the 
brain, in which in the higher types, two regions may be distinguished, 
viz. an alisphenoidal region behind, growing out from what is known 
as the basisphenoidal region of the primitive trabecular, and an 
orbito-sphenoidal region in front, growing out from the presphenoidal 
region of the trabecuhe. These plates form at first a continuous lateral 
wall of the cranium. The cartilaginous walls which grow up from the 
trabecular floor of the cranium generally extend upwards so as to form 
a roof, though almost always an imperfect roof, for the cranial cavity. 
The basi-cranial cartilaginous skeleton reduces itself always into 
trabecuhe and parachordals with olfactory and auditory cartilaginous 
capsules. 



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THE EVIDENCE OF THE SKELETON 



123 



In addition, a branchial skeleton exists, which consists of a series 
of bars known as the branchial bars, so situated as to afford support 
to the successive branchial pouches. An anterior arch known as the 
mandibular arch (Fig. 50, 
Mn.\ placed in front of the 
hyo-mandibular cleft, and 
a second arch, known as the 
hyoid arch {Hy.) t placed in 
front of the hyo-branchial 
cleft, are developed in all 
types ; the succeeding arches 
are known as the true bran- 
chial arches (Br.) } and are 
only fully developed in the 
Ichthyopsida. In all cases 
of jaw-bearing (gnathosto- 
matous) vertebrates the first 
arch has become a support- 
ing skeleton for the mouth (Fig. 51), and in the higher vertebrates in 
combination with the second or hyoid arch, takes part in the formation 
of the ear-bones. 

The true branchial arches persist, to a certain extent, in the 




Fig. 60.— Head op Embryo Dog-fish, eleven 
lines long. (From Parker.) 

Tr. t trabecula; Mn., mandibular cartilage ; iff/., 
hyoid arch; Br,., first branchial arch; Na. } 
olfactory sac ; E., eye ; An., auditory capsule ; 
Hm. t hemisphere; C,, C 2 , C 3 , cerebral vesicles. 




Hu'Hy 



Fig. 51.— Skull of Adult Dog-fish, Side View. (From Parker.) 
cr. t cranium ; Br., branchial arches ; 3/n. + i/»/., mandibular and hyoid arches. 

Amphibia, and become still more degenerated in the Amniota 
(reptiles, birds, and mammals) in con-elation with the total dis- 
appearance of a branchial respiration at all periods of their life. 



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124 THE ORIGIN OF VERTEBRATES 

Their remnants become more or less important parts of the hyoid 
bone, and are employed solely in support of the tongue. 

In no single animal is there any evidence that the foremost arch, 
the mandibular, is a true branchial arch. As low down as the 
Elasmobranchs it becomes divided into two elements which form 
respectively the upper and lower jaws ; the hyoid arch, on the other 
hand, although it has altered its form and acquired the secondary 
function of supporting the mandibular arch, still retains its respi- 
ratory function. 

.The evidence afforded by the mode of formation of the skeletal 
tissues of vertebrates down to the Elasmobranchs indicates that the 
primitive cranial skeleton arose from two paired basal cartilages, the 
parachordals and trabecule, to which were attached respectively 
cartilaginous cases enclosing the organs of hearing and smell. In 
addition, the branchial portion of the cranial region was provided 
with cartilaginous bars arranged serially for the support of the 
branchiae, with the exception of the foremost, the mandibular bar, 
which formed supporting tissues for the mouth — the upper and 
lower jaws. 

Just as in past times the spinal nerves and the segments they 
supplied were supposed to represent the type on which the original 
vertebrate was built, so also the spinal vertebrae afforded the type of 
the segmented skeleton, and the anatomists of those days strove hard 
to resolve the cranio-facial skeleton into a series of modified vertebrae. 
Owing especially to the labours of Huxley, who showed that the seg- 
mentation in the head-region was essentially a segmentation due to 
the presence of branchial bars, this conception was finally laid to rest 
and nowadays it is admitted to be hopeless to resolve the cranium 
into vertebral segments. Still, however, the vertebrate is a segmented 
animal and its segmented nature is visible in the cranial region, so far 
as the skeletal tissues are concerned, in the shape of the series of 
branchial and visceral bars. 

To this segmentation the name of ' branchiomeric ' has been given, 
while that due to the presence of vertebrae is called ' mesomeric/ 

As we have seen, the internal bony skeleton of the vertebrate 
commences as a cartilaginous and membranous skeleton. For this 
reason the preservation of such skeletons is impossible in the fossil 
form, unless the cartilage has become impregnated with lime salts, 
so that there is but little hope of ever obtaining traces of such 



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THE EVIDENCE OF THE SKELETON 1 25 

structures in the fossils of the Silurian age either among the verte- 
brate or invertebrate remains. Fortunately for this investigation 
there are still living on the earth two representatives of that age ; on 
the invertebrate side Limulus, and on the vertebrate side Ammocoetes. 

The Elasmobranchs represent the most primitive of the gnatho- 
stomatous vertebrates. Below them come the Agnatha, known as the 
cyclostomatous fishes or Marsipobranchii, the lampreys (Petromyzon) 
and the hag-fishes (Myxine). 

The skeleton of Petromyzon (Fig. 52) consists of a cranio-facial 
skeleton composed of a cartilaginous unsegmented cranium, with the 
basal trabecule and parachordals and a series of branchial and visceral 
cartilaginous bars forming the so-called branchial basket-work; to 
these must be added auditory and nasal capsules. In contradis- 
tinction to this elaborate cranio-facial skeleton, the spinal vertebral 




Fig. 52.— Skeleton op Petromyzon. (From Parker.) 
na. t nasal capsule ; au., auditory capsule ; nc. f notochord. 

skeleton is represented only by segmentally arranged small pieces of 
cartilage formed in the connective tissue dissepiments between 
segmented sheets of body-muscles (myotomes). 

But Petromyzon is derived from Ammoccetes by a remarkable 
process of transformation, and a most important part of that trans- 
formation is the formation of new cartilaginous structures. Thus we 
see that in Ammoccetes there is no sign of a cartilaginous vertebral 
column ; at transformation the rudimentary vertebra} of Petromyzon 
are formed. In Amnioccetes the brain-case is a simple fibrous mem- 
branous covering ; at transformation this becomes cartilaginous. In 
Ammoccetes there are no cartilaginous structures corresponding to 
the sub-ocular arches; these are all formed at transformation. It 
follows, that we can trace back the bony skeleton of the vertebrate 
head to the skeleton of Ammoccetes, and we may therefore conclude 



-— ^J 



126 



THE ORIGIN OF VERTEBRATES 



that the primitive cartilaginous skeleton of the vertebrate consisted 
of the following structures (Fig. 53, B), viz. the branchial bars 

forming a basket-work, the 
trabecule and parachordals, 
the auditory and nasal cap- 
sules — a clear proof that the 
cranial skeleton is older than 
the spinal. Of these struc- 
tures the branchial bars are 
the only evidently segmented 
parts. 




PI 



Ent 



s 
s 



V 




The Soft Cartilage of the 
Branchial Skeleton of 
Ammocgetes. 



O— nc 



The study of Ammocoetes 
gives yet another clue to the 
nature of the earliest skeleton, 

jm ^£ V-^'V/^-Y ^ or ^ ese two mar ^ e( i groups 

&\ \\ ^ °\ i W M i of cartilage — the branchial and 

basi-cranial — are characterized 
by a difference in structure as 
well as a difference in topo- 
graphical position. J. Miiller 
was the first to point out that 
these two sets of cartilages 
differ in appearance and con- 
stitution, and he gave to them 
the name of yellow and grey 
cartilage. Parker has described 
them fully under the terms 
soft and hard cartilage, terms 
which Schaffer has also used, 
and I shall also make use of 
them here. The whole of the 
branchial cartilaginous skele- 
ton is composed of soft cartilage, while the basi-cranial skeleton, con- 
sisting of trabecular, parachordals, and auditory capsule, is composed 



Fig. 53.— Comparison of Cartilaginous 
Skeleton of Limulus and Ammoccetes. 

A, Diagram of cartilaginous skeleton of 
Limulus. Soft cartilage, entapophysial liga- 
ments, deep black; branchial bars simply 
hatched; Jiard cartilage, lateral trabecules 
of entosternite, netted; Ph., position of 
pharynx. 

B, Diagram of cartilaginous skeleton of 
Ammocoetes. Soft cartilage, sub-chordal 
cartilaginous bands, deep black; branchial 
basket-work (first formed part), simply 
hatched ; hard cartilage, cranio-facial skele- 
ton, trabecular, parachordals and auditory 
capsules, netted; Inf., position of tube of 
infundibulum (old oesophagus). 



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THE EVIDENCE OF THE SKELETON 1 27 

of hard cartilage, the only soft cartilage in this region being that 
which forms the nasal capsule, not represented in Fig. 53, B. 

These two groups of cartilage arise independently, so that at first 
the basi-cranial system is quite separate from the branchial, and only 
late in the history of the animal is a junction effected between the 
branchial system and the trabecule and parachordals, an initial 
separation wliich is especially striking when we consider that in this 
animal all the cartilaginous structures of any one system are con- 
tinuous : there is no sign of anything in the nature of joints. 

Of these two main groups, the branchial cartilages are formed first 
in the embryo, a fact which suggests that they are the most primi- 
tive of the vertebrate cartilages, and that, therefore, the first true 
formation of cartilage in the invertebrate ancestor may be looked for in 
the shape of bars supporting the branchial mechanism. The evidence 
of the origin of the cartilaginous structures in Ammoccetes is given 
by Shipley in the following words : — 

" The branchial bases are the first part of the skeleton to appear. 
They arise about the 24th day as straight bars of cartilage, lying 
external and slightly posterior to the branchial vessel. 

" The first traces of the basi-cranial skeleton appear on the 30th 
day as two rods of cartilage — the trabecule." 

Our attention must, in the first place, be directed to this branchial 
basket-work of Ammocoetes. 

Underlying the skin of Ammoccetes in the branchial region is 
situated the sheet of longitudinal body-muscles, divided into a series 
of segments or myotomes, which forms the somatic muscles so cha- 
racteristic of all fishes. This muscular sheet is depicted on the left- 
hand side of Fig. 54. It does not extend over the lower lip or over 
that part in the middle line where the thyroid gland is situated. In 
these parts a sheet of peculiar tissue known by the name of muco- 
cartilage lies immediately under the skin, covering over the thyroid 
gland and lower lip. The somatic muscular sheet with the super- 
jacent skin can be stripped off very easily owing to the vascularity 
and looseness of the tissue immediately underlying it. When this is 
done the branchial basket-work comes beautifully into view as is 
seen on the right-hand side of Fig. 54. It forms a cage within which 
the branchiae and their muscles lie entirely concealed. 

This is the great characteristic of this most primitive form of the 
branchial cartilaginous bars and distinguishes it from the branrhial 



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128 



THE ORIGIN OF VERTEBRATES 



bars of other higher fishes, in that it forms a system of cartilages 
which lie external to the branchiae — an extra-branchial system. 

This branchial basket-work is simpler in Ammoccetes than in 
Petrorayzon, and its actual starting-point consists of a main trans- 
verse bar corresponding to each branchial 
segment; from this transverse bar the 
system of longitudinal bars by which 
the basket-work is formed has sprung. 
These transverse bars arise from a 
cartilaginous longitudinal rod, situated 
close against the notochord on eacli 
side. These rods may be called the 
subchordal cartilaginous bands (Fig. 53), 
and, according to the observations 6f 
Schneider and others, each subchordal 
band does not form at first a continuous 
cartilaginous rod, but the cartilage is 
conspicuous only at the places where 
the transverse bars arise. In the 
youngest Ammoccetes examined by 
Schaffer, he could find no absolute dis- 
continuity of the cartilage except be- 
tween the first two transverse bars, but 
he says that the thinning between the 
transverse bars was so marked as to 
make it highly probable that • at an 
earlier stage there was discontinuity. 
The whole system of branchial bars and 
subchordal rods is at first absolutely 
disconnected from the cranial system of 
trabecule and parachordals, and only 
later do the two systems join. 

These observations on Ammoccetes 
lead most definitely to the conclusion 
that the starting-point of the whole cartilaginous skeleton of 
the vertebrate consisted of a series of transverse cartilaginous bars, 
for the purpose of supporting branchial segments ; these were con- 
nected with two axial longitudinal cartilaginous rods, which at first 
contained cartilage only near the places of junction of the branchial 




Fig. 64.— Ventral View of 
Head Region op Ammocostes. 

thyroid gland; M. t lower 
lip, with its muscles. 



Th 



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THE EVIDENCE OF THE SKELETON 1 29 

bars. This system may be called the mesosomatic skeleton, as it is 
entirely confined to the branchial or mesosomatic region. 

In addition to this primitive cartilaginous framework, which was 
formed for the support of the mesosomatic or respiratory segments, 
but at a slightly later period in the phylogenetic history, a separate 
cartilaginous system was formed for the support of the prosomatic 
segments, viz. the trabecule and parachordals with the auditory cap- 
sules : a system which was at first entirely separated from the mesoso- 
matic, and, as we shall see, is more advanced in structure than the 
branchial system*. Later still, the story is completed at the time of 
transformation to Petromyzon by the formation of the simple cartila- 
ginous skull and the rudimentary vertebrae, the structure of which 
is also of a more advanced type. 

The Structure of the Soft Branchial Cartilage. 

Having considered the topographical position of the primitive 
branchial cartilaginous skeleton, we may now inquire, What was 
its structure and how was it formed ? 

In the higher vertebrates various forms of cartilage are described, 
viz. hyaline, fibro-cartilage, elastic cartilage, and parenchymatous 
cartilage. Of these, the parenchymatous cartilage is looked upon as 
the most primitive form, because it preserves without modification 
the characters of embryonic cartilage. 

Embryology, then, would lead to the belief that the earliest form 
of cartilage in the vertebrate kingdom ought to be of this type, viz. 
large cells, each of which is enclosed in a simple capsule, so that the 
capsules of the cells form the whole of the matrix, and thus form a 
simple homogeneous honeycomb-structure, in the alveoli of which 
the cartilage-cells lie singly. If, then, the branchial cartilages of 
Ammocoetes are, as has just been argued, the representatives of the 
cartilaginous skeleton of the primitive vertebrate, it is reasonable to 
suppose that they should resemble in structure this embryonic car- 
tilage. Such is undoubtedly the case : all observers who have 
described the branchial basket-work of Ammocoetes or Petromyzon 
have been struck with the extremely primitive character of the car- 
tilage, and the last observer (Schaffer) describes it as composed of 
thin walls of homogeneous material, in which there are no lines of 
separation, which form a simple honeycomb-structure, in the alveoli 

K 



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130 THE ORIGIN OF VERTEBRATES 

of which the separate cells lie singly. These branchial cartilages are 
each surrounded by a layer of perichondrium, and in Fig. 55, A, I 
give a picture of a section of a portion of one of the bars. 





Fig. 55.— A, Branchial Cartilage op Ammocxetes, stained with Thionin. B, 
Branchial Cartilage of Limulus, stained with Thionin. 

Hence we see that structurally as well as topographically the 
branchial bars of Ammoccetes justify their claim to be considered as 
the origin of the vertebrate cartilaginous framework. 



On the Structure of the Muco-cartilage in Ammoccetes. 

We can, however, go further than this, and ask how this cartilage 
itself is formed in Ammoccetes ? The answer is most definite, most 
instructive and suggestive, for in all cases this particular kind of car- 
tilage is formed from, or at all events in, a peculiar fibrous tissue, 
which was called by Schneider " Schleim-Knorpel,'* or muco-cartilage, 
a tissue which is distinguishable from other connective tissues, not 
only by its structural peculiarities, but also by its strong affinity for 
all dyes which differentiate mucoid or chondro-mucoid substances. 

This muco-cartilage is thus described by Schneider: — The peri- 
chondrium in Ammoccetes is not confined to the true cartilaginous 
structures, but extends itself in the form of thin plates in definite 
directions. Between these plates of perichondrium a peculiar tissue 
(Fig. 56) — the muco-cartilage — exists, consisting of tibrilke, whose 
direction is mainly at right angles to the planes of the perichondrial 
plates, with star-shaped cells in among them, and with the spaces 
between the fibrilhe filled up with a semi-fluid mass. 



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THE EVIDENCE OF THE SKELETON 131 

From this tissue all the primitive cartilages which resemble the 
branchial bars are formed, either by the invasion of chondroblasts 
from the surrounding perichondrium, or by the proliferation and 
encapsulation of the cells of the muco- cartilage itself. 

This very distinctive tissue — the muco-cartilage — is of very great 
importance in all questions of the origin of the skeletal tissues. In 
all descriptions of the skeletal tissues it has been practically dis- 
regarded until recent years when, besides my own observations, its 
distribution has been mapped out by Schaffer. Thus Parker, in his 
well-known description of the skeleton of the marsipobranch fishes, 
does not even mention its existence. Its importance is shown by its 
absolute disappearance at 

transformation and its non- ^,^^^&*^ Z: z 

occurrence in any of the ^^^-"^^^^^frf^^t ?J^ 
higher vertebrates. It is **" NiT^n i' ly M?[J!4 'mP'^ 
entirely confined to the head- ♦Tw'n \ \ J J l_ At \ y^~\ ^A\ 

region, and its distribution 
there is most suggestive, for, i« \ 
as will be described fully fy^ 
later on, it forms a skele- 
ton which both in structure 

and position resembles very I !C^f H^j-j l^^f 

closely the head-shields of 
cephalaspidian fishes. At 

the present part of my argu- ^-^—^ V_ 

ment its more immediate Fig. 66.— Section op Muco-cartilage from 
interest lies in the method Dorsal Head-plate op Ammoc<etes. 

of tracing this tissue. For 

this purpose I made use of the micro- chemical reaction of thionin, 
a dye which, as shown by Hoyer, stains all mucin-containing sub- 
stances a bright purple. Schaffer made use of a corresponding 
basophil stain, haemalum. When stained with thionin, the matrix, 
or ground-substance of the branchial cartilages as well as the matrix 
or semi-fluid substance in which the fibrils of the muco-cartilaginous 
cells are embedded take on a deep purple colour, while the fibrous 
material of the cranial walls and other connective tissue strands, such 
as the perichondrium, are coloured light blue. Muco-cartilage, then, 
may be described as a peculiar form of connective tissue which 
differs from other connective tissue not only in its appearance but in 







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132 THE ORIGIN OF VERTEBRATES 

its chemical composition, for unlike white fibrous tissue it contains a 
large amount of mucin, and this tissue is the forerunner of the earliest 
cartilaginous vertebrate skeleton, the branchial bars of Ammoccetes. 

The conclusions to which we are led by the study of the structure, 
position, and mode of origin of these primitive cartilages of 
Ainmoccete8 may be thus summed up : — 

1. The immediate ancestor of the vertebrate must have possessed 
a peculiar fibrous tissue — the ground-substance of which stained deep 
purple with thionin — in which cartilage arose. 

2. The cartilage so formed was not like hyaline cartilage, but 
resembled in a striking manner parenchymatous cartilage. 

3. This cartilage was situated partly in two axial longitudinal 
bands, partly as transverse bars, which supported the brancliial 
apparatus. 

The Prosomatic or Basi-cranial Skeleton of Ammoccetes. 

Before searching for any evidence of a similar tissue in any 
invertebrate group, it is advisable to consider the other portion of 
the cartilaginous skeleton of Ammocoetes, which consists of the tra- 
becule, parachordals and auditory capsules — the basi-cranial skeleton 
— and is composed of hard, not soft cartilage. 

This basi-cranial skeleton represented in Fig. 53, B, is confined to 
the region of the notochord, the cranial walls being composed entirely 
of a white fibrous membrane. It is separated at first entirely from 
the sub-chordal portion of the branchial basket-work, and is com- 
posed of a foremost part, the trabecule (Tr.), and of a hindermost 
part, the parachordals (Pr.cA.), which are characterized by the 
attachment on each side of the large auditory capsule (Au.). In 
Ammoccetes the trabecular bars are continuous with the parachordals, 
the junction being marked by a small lateral projection on each side, 
which at transformation is seen to play an important part in the 
formation of the sub-ocular arch. The trabecular bar lies close 
against the notochord on each side up to its termination; it then 
bends away from the middle line and curves round until it meets 
its fellow on the opposite side, thus forming, as it were, the head of 
a racquet of which the notochord forms the splice in the handle. 
The strings of the racquet are represented by a thin membrane, in 
the centre of which the position of the infundibulum {Inf.) of the 



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THE EVIDENCE OF THE SKELETON 



133 



brain can be clearly seen. In an earlier stage of Ammocoetes the 
two trabecular horns do not meet, but are separated by connective 
tissue, which afterwards becomes cartilaginous. 

As far, then, as the topography of this basi-cranial skeleton is 
concerned, the striking jfbints are — the shape of the trabecular 
portion, diverging as it does around the infundibulum, and the pre- 
sence on the parachordal portion of the two large auditory capsules. 

These two points indicate, on the hypothesis that infundibulum 
and oesophagus are convertible terms, that two supporting structures 
of a cartilaginous nature must have existed in the ancestor of the 
vertebrate, the first of which surrounded the oesophagus, and the 
second was in connection with its auditory apparatus. 

Structure of the Hard Cartilages. 

The structure of this hard cartilage of the trabecule and auditory 
capsules resembles that of the soft, in so far that it consists of large 





A 

Fig. 57.— A, Cartilage op Trabecule of Ammoccetes, stained with Hema- 
toxylin and Picric Acid. B, Nests of Cartilage Cells in Entosternite 
of Hypoctonus, stained with Hematoxylin and Picric Acid. 

cells with a comparatively small amount of intercellular substance. 
Schaffer, who has described it lately, considers that it is a nearer 
approach to hyaline cartilage than the soft, but yet cannot be called 
hyaline cartilage in the usual sense of the term. Its peculiarities 
and its differences from the soft are especially well seen by its 
staining reactions. I have myself been particularly struck with 
the effect of picrocarmine or combined hematoxylin and picric acid 



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134 THE ORIGIN OF VERTEBRATES 

staining (Fig. 57). In the case of the soft cartilage the capsular 
substance stains respectively a brilliant red or blue, while that of 
the hard cartilage is coloured a deep yellow, so that the junction 
between the parachordals and the branchial cartilages is beautifully 
marked out. Then, again, with thionin* which gives so marked a 
reaction in the case of the soft cartilage, the hard cartilage of the 
auditory capsule is not stained at all, and in the trabecule the deep 
purple colour is confined to the mucoid cement-substance between 
the capsules, just as Schaffer has stated. The same kinds of reactions 
have been described by Schaffer: thus by double staining with 
heemalum-eosin the hard cartilage stains red, the soft blue ; and he 
points out that even with over-staining by haemalum the auditory 
capsule remains colourless, just as I have noticed with thionin. He 
infers, precisely as I have done from the thionin reaction, that 
chondro-mucoid, which is so marked a constituent of the soft cartilage 
and of the muco-cartilage, is absent or present in but slight quantities 
in the hard cartilage. Similarly, he points out that double staining 
with tropoeolin-methyl- violet stains the hard cartilage a bright orange 
colour, and the soft cartilage a violet. 

The evidence, then, shows clearly that a marked chemical differ- 
ence exists between these two cartilages, which may be expressed by 
saying that the one contains very largely a basophil substance, 
which we may speak of as belonging to the class of chondro-mucoid 
substances, while the other contains mainly an oxyphil substance, 
probably a chondro-gelatine substance. 

We may perhaps go further and attribute this difference of 
composition to a difference of origin ; for whereas the soft cartilage 
is invariably formed in a special tissue, the muco-cartilage, which 
shows by its reaction how largely it is composed of a mucoid sub- 
stance, the hard cartilage is certainly, in the case of the cartilage of 
the cranium where its origin has been clearly made out, formed in 
the membranous tissue of the cranium of Ammoccetes — i.e. in a 
tissue which stains light blue with thionin, and contains a gelatinous 
rather than a mucoid substratum. 

The best opportunity of finding out the mode of origin of the 
hard cartilage is afforded at the time of transformation, when so 
much of this kind of cartilage is formed anew. Unfortunately, it 
is very difficult to obtain the early transformation stages, conse- 
quently we cannot be said to possess any really exhaustive and 

S V 

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THE EVIDENCE OF THE SKELETON 1 35 

definite account of how the new cartilages are formed. Bujor, 
Kaensche, and Schaffer all profess to give a more or less definite 
account of their formation, and the one striking impression left on 
the mind of the reader is how their descriptions vary. In one 
point only are they agreed, and in that I also agree with them, viz. 
the manner in which the new cranial walls are formed. Schaffer 
describes the 'process as the invasion of chondroblasts into the 
homogeneous fibrous tissue of the cranial walls. Such chondro- 
blasts not only form the cartilaginous framework, but also assimilate 
the fibrous tissue which they invade, so that finally all that remains 
of the original fibrous matrix in which the cartilage was formed are 
these lines of cement-substance between the groups of cartilage 
cells, which, containing some basophil material, are marked out, as 
already mentioned (Fig. 57). 

We may therefore conclude, from the investigation of Ammoccetes, 
that the front part of the basi-cranial skeleton arose as two trabecular 
bars, to which muscles were attached, situated bilaterally with respect 
to the central nervous system. These bars were composed of tendinous 
material with a gelatinous rather than a mucoid substratum, in which 
nests of cartilage-cells were formed, the cartilaginous material formed 
by these cells being of the hard variety, not staining with thionin, 
and staining yellow with picro-carmine, etc. By the increase of such 
nests and the assimilation of the intermediate fibrous material, the 
original fibro-cartilage was converted into the close-set semi-hyaline 
cartilage of the trabecule and auditory capsules, in which the fibrous 
material still marks out by its staining-reaction the limits of the 
cell-clusters. 

Such I gather to be Schaffer's conclusions, and they are certainly 
borne out by my own and Miss Alcock's observations. As far as 
we have had an opportunity of observing at present, the first process 
at transformation appears to consist of the invasion of the fibrous 
tissue of the cranial wall by groups of cells which form nests of cells 
between the fibrous strands. These nests of cells form round them- 
selves capsular material, and thus form cell-territories of cartilage, 
which squeeze out and assimilate the surrounding fibrous tissue, until 
at last all that remains of the original fibrous matrix is the lines of 
cement-substance which mark out the limits of the various cell-groups. 

At present I am inclined to think that both soft and hard cartilage 
originate in a very similar manner, viz. by the formation of capsular 



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136 THE ORIGIN OF VERTEBRATES 

material around the invading chondroblasts, and that the difference 
in the resulting cartilage is mainly due to the difference in chemical 
composition of the matrix of the connective tissue which is invaded. 
Thus the difference may be formulated as follows : — 

The hard cartilage is formed by the invasion of chondroblasts 
into a fibrous tissue, which contains a gelatinous rather than a mucoid 
substratum, in contradistinction to the soft cartilage which is formed, 
probably also by the invasion of chondroblasts, in a tissue — the 
muco-cartilage — which contains a specially mucoid substratum. 

Such, then, is the very clearly defined starting-point of the ver- 
tebrate skeleton — two distinct formations of different histological 
and chemical structure,— the one forming a segmented branchial 
skeleton, the other a non-segmented basi-cranial skeleton. 

The Cartilaginous Skeleton of Limulus. 

Among the whole of the invertebrates at present living on the 
earth, is there any sign of an internal cartilaginous skeleton that 
will give a direct clue to the origin of the primitive vertebrate 
skeleton ? The answer to this question is most significant : only 
one animal among all those at present known possesses a cartilaginous 
skeleton, which is directly comparable with that of Amnioccetes, and 
here the comparison is very close — only one animal among the 
thousands of living invertebrate forms, and that animal is the only 
representative still surviving of the palaeostracan group, which was 
the dominant race when the vertebrate first made its appearance. 
The Limulus, or king-crab, possesses a segmented branchial internal 
cartilaginous skeleton (Fig. 53, A), made up of the same kind of cartilage 
as the branchial skeleton of Ammoccetes, confined to the mesosomatic 
or branchial region, just as in Ammoccetes, forming, as in Ammocoetes, 
cartilaginous bars supporting the branchiae, and these bars are situated 
externally to the branchiae, as in Ammocoetes. In addition this 
animal possesses a basi-cranial internal semi-cartilaginous unseg- 
mented plate known as the entosternite or plastron situated, with 
respect to the oesophagus, similarly to the position of the trabeculae 
with respect to the infundibulum in Ammoccetes. Moreover, the 
cartilaginous cells in this tissue differ from those in the branchial 
region, in precisely the same manner as the hard cartilage differs from 
the soft in Ammocoetes. 



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THE EVIDENCE OF THE SKELETON 1 37 

This plastron, it is true, is found in other animals, all of which 
are members of the scorpion tribe, except in one instance, and this, 
strikingly enough, is the crustacean Apus — a strange primitive form, 
which is acknowledged to be the nearest representative of the 
Trilobita still living on the earth. None of these forms, however, 
possess any sign of an internal cartilaginous branchial skeleton, 
such as is possessed by Limulus. Scorpions, Apus, Limulus, are 
all surviving types of the stage of organization which had been 
reached in the animal world when the vertebrate first appeared. 

The Mesosomatic or Respiratory Skeleton of Limulus, composed 
of Soft Cartilage. 

Searching through the literature of the histology of the cartila- 
ginous tissues in invertebrate animals, to see whether any cartilage 
had been described similar to that seen in the branchial cartilages of 
Ammoccetes, and whether such cartilage, if found, arose in a fibrous 
tissue resembling muco-cartilage, I was speedily rewarded by finding, 
in Ray Lankester's article on the tropho-skeletal tissues of Limulus, 
a picture of the cartilage of Limulus, which would have passed muster 
for a drawing of the branchial cartilage of Ammoccetes. This clue 
I followed out in the manner described in my former paper in the 
Journal of Anatomy and Physiology, and mapped out the topography 
of this remarkable tissue. 

Limulus, like other water-dwelling arthropods, breathes by means 
of gills attached to its appendages. These gill-bearing appendages 
are confined to the mesosomatic region, as is seen in Fig. 59 ; and these 
appendages are very different to the ordinary locomotor appendages, 
which are confined to the prosomatic region. Each appendage, as is 
seen in Fig. 58, consists mainly of a broad, basal part, which carries 
the gill-book on its under surface ; the distal parts of the appendage 
have dwindled to mere rudiments and still exist, not for locomotor 
purposes, but because they carry on each segment organs of special 
importance to the animal (see Chapter XI.). As is seen in Fig. 58, 
the basal parts of each pair of appendages form a broad, flattened 
paddle, by means of which the animal is able to swim in a clumsy 
fashion. Very striking and suggestive is the difference between 
these gill-bearing mesosomatic appendages and the non-gill-bearing 
locomotor appendages of the prosoma. 



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i3« 



THE ORIGIN OF VERTEBRATES 



At the base of each of these appendages, where it is attached to 
the body of the animal, the external chitinous surface is characterized 



Br,C. En V Dy ^jJLj± N. E f A LV.S. 




Fig. 58. — Transverse Section through the Mesosoma op Limulus, to show 
the Anterior (A) and the Posterior (B) Surfaces of a Mesosomatic or 
Branchial Appendage. 

In each figure the branchial cartilaginous bar, Br.C, has been exposed by dissection 
on one side. Ent., entapophysis ; Ent. I., entapophysial ligament cut across; 
Br. C.j branchial cartilaginous bar, which springs from the entapophysis; if., 
heart; P., pericardium; Al. t alimentary canal; N., nerve cord; L.T.S., longi- 
tudinal venous sinus ; Dv. } dorso-ventral somatic muscle; Vp., veno-pericardial 
muscle. 

by a peculiar stumpy, rod-like marking, and upon removing the 
chitinous covering, this surface-appearance is seen to correspond to a 
well-marked rod of cartilage (Br.C), which extends from the body 



s* "V 



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THE EVIDENCE OF THE SKELETON 1 39 

of the animal well into each appendage. This bar of cartilage arises 
on each side from the corresponding entapophysis (Ent.), which is 
the name given to a chitinous spur which projects a short distance 
(Fig. 58, B) into the animal from the dorsal side, for the purpose of 
giving attachment to various segmental muscles. These entapophyses 
are formed by an invagination of the chitinous surface on the dorsal 
side and are confined to the mesosomatic region, so that the meso- 
somatic carapace indicates, by the number of entapophyses, the 
number of segments in that region, in contradistinction to the pro- 
somatic carapace, which gives no indication on its surface of the 
number of its components. 

Each entapophysis is hollow and its walls are composed of chitin ; 
but from the apex of each spur there stretches from spur to spur 
a band of tissue, called by Lankester the entapophysial ligament 
(EnU.) (Fig. 58), and in this tissue cartilage is formed. Isolated 
cartilaginous cells, or rather groups of cells, are found here and there, 
but a concentration of such groups always takes place at each enta- 
pophysis, forming here a solid mass of cartilage, from which the 
massive cartilaginous bar of each branchial appendage arises. 

Further, not only is this cartilage exactly similar to parenchy- 
matous cartilage, as it occurs in the branchial cartilages of Animoccetes, 
but also its matrix stains a brilliant purple with thionin in striking 
contrast to the exceedingly slight light- blue colour of the surrounding 
perichondrium. In its chemical composition it shows, as might be 
expected, that it is a cartilage containing a very large amount of 
some mucin-body. 

The Muco- cartilage of Limulus. 

The resemblance between this structure and that of the branchial 
bars of Ammoccetes does not end even here, for, as already mentioned, 
the cartilage originates in a peculiar connective tissue band, the 
entapophysial ligament, and this tissue bears the same relation in 
its chemical reactions to the ordinary connective tissue of Limulus, 
as muco-cartilage does to the white fibrous tissue of Ammoccetes. 
The white connective tissue of Limulus, as already stated, resembles 
that of the vertebrate more than does the connective tissue of any 
other invertebrate, and, similarly to that of Ammoccetes, does not 
stain, or gives only a light-blue tinge with thionin. The tissue of 



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140 



THE ORIGIN OF VERTEBRATES 



the entapophysial ligament, on the contrary, just like rauco-cartilage, 
takes on an intense purple colour when stained with thionin. It 
possesses a mucoid substratum, just as does muco-cartilage, and in 
both cases a perfectly similar soft cartilage is born from it. 

One difference, however, exists between the branchial cartilages of 
these two animals ; the innermost axial layer of the branchial bar of 




-6 



Fig. 59.— Diagram op Limulus, to show the Nerves to the Appendages (1-13) 
and the Branchial Cartilages. 

The branchial cartilages and the entapophysial ligaments arc coloured blue, the 
branchiae red. gl., generative and hepatic glands surrounding the central nervous 
system and passing into the base of the flabellum (fl.). 

Limulus is very apt to contain a specially hard substance, apparently 
chalky in nature, so that it breaks up in sections, and gives the 
appearance of a broken-down spongy mass ; if, however, the tissue is 
first placed in a solution of hydrochloric acid, it then cuts easily, and 
the whole tissue is seen to be of the same structure throughout, the 
main difference being that the capsular spaces in the axial region 
are much larger and much more free from cell-protoplasm than are 
those of the smaller younger cells near the periphery. 



^ ^\ 



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THE EVIDENCE OF THE SKELETON 



141 



I have attempted in Fig. 53 to represent this close resemblance 
between the segmented branchial skeleton of Limulus and of Amino- 
coutes, a resemblance so close as to reach even to minute details, 
such as the thinning out of the cartilage in the subchordal bands and 




Fig. 60.— Diagram of Ammooetes cut open to show the Lateral System of 
Cranial Nerves V., VII., IX., X., and the Branchial Cartilages. 

The branchial cartilages and sub-chordal ligaments are coloured blue, the branchiue 
red. gl. t glandular substance surrounding the central nervous system and pass- 
ing into the auditory capsule with the auditory nerve (VIII.). 

entapophysial ligaments respectively between the places where the 
branchial bars come off. 

In Fig. 59 I have shown the prosoma and mesosoma of Limulus, 
and indicated the nerves to the appendages together with the meso- 
somatic cartilaginous skeleton. 

In Fig. 60 I have drawn a corresponding picture of the prosomatic 
and mesosomatic region of Ammocoetes with the corresponding nerves 



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142 THE ORIGIN OF VERTEBRATES 

and cartilages. In this figure the animal is supposed to be slit open 
along the ventral mid-line and the central nervous system exposed. 



The Prosomatic Skeleton of Limulus, composed of Hard 

Cartilage. 

The rest of the primitive vertebrate skeleton arose in the proso- 
matic region, and formed a support for the base of the brain. This 
skeleton was composed of hard cartilage, and arose in white fibrous 
tissue containing gelatin rather than mucin. 

Is there, then, any peculiar tissue of a cartilaginous nature in 
Limulus and its allies, situated in the prosomatic region, which is 
entirely separate from the branchial cartilaginous skeleton, which 
acts as a supporting internal framework, and contains a gelatinous 
rather than a mucoid substratum ? 

It is a striking fact, common to the whole of the group of animals 
to which our inquiries, deduced from the consideration of the structure 
of Ammoccetes, have, in every case, led us in our search for the verte- 
brate ancestor, that they do possess a remarkable internal semi-carti- 
laginous skeleton in the prosomatic region, called the entosternite or 
plastron, which gives support to a large number of the muscles of 
that region ; which is entirely independent of the branchial skeleton, 
and differs markedly in its chemical reactions from that cartilage, in 
that it contains a gelatinous rather than a mucoid substratum. 

In Limulus it is a large, tough, median plate, fibrous in character, 
in which are situated rows and nests of cartilage-cells. The same 
structure is seen in the plastron of Hypoctonus, of Thelyphonus, 
and to a certainty in all the members of the scorpion group. Very 
different is the behaviour of this tissue to staining from that of the 
branchial region. No part of the plastron stains purple with 
thionin; it hardly stains at all, or gives only a very slight blue 
colour. In its chemical composition there is a marked preponder- 
ance of gelatin with only a slight amount of a mucin-body. In 
some cases, as in Hypoctonus (Fig. 57, B) and Mygale, the capsules 
of the cartilage-cells stain a deep yellow with hematoxylin and 
picric acid, while the fibres between the cell-nests stain a blue-brown 
colour, partly from the hematoxylin, partly from the picric acid. 

All the evidence points to the plastron as resembling the basi- 
cranial skeleton of Ammoccetes in its composition and in the origin 



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THE EVIDENCE OF THE SKELETON 



1 43 



of its cells in a white fibrous tissue. What, then, is its topographical 
position ? It is in all cases a median structure lying between the 
cephalic stomach and the infra-cesophageal portion of the central 
nervous system, and in all cases it possesses two anterior horns which 
pass around the oesophagus and the nerve-masses which immediately 
enclose the oesophagus (Fig. 61, A). These lateral horns, then, 
which lie laterally and slightly ventral to the central nervous 
system, and are called by Bay Lankester and Benham the sub- 
neural portion of the entosternite, are very nearly in exactly the 
position of the racquet- shaped head of the trabecule in Ammoccetes. 
It is easy to see that, with a more extensive growth of the nervous 
material dorsally, such lateral 
horns might be caused to take 
up a still more ventral posi- 
tion. Now, these two lateral 
horns of the plastron of Li- 
uiulus are continued along 
its whole length so as to form 
two thickened lateral ridges, 
which are conspicuous on the 
flat surface of the rest of this 
median plate. In other cases, 
as in the Thelyphonidie, the 
plastron consists mainly of 
these two lateral ridges or 
trabecule, as they might be 
called, and Schimkewitsch, 
who more than any one else has made a comparative study of the 
entosternite, describes it as composed in these animals of two lateral 
trabeculie crossed by three transverse trabecule. I myself can con- 
firm his description, and give in Fig. 61, B, the appearance of the 
entosternite of Thelyphonus or of Hypoctonus. The supra-cesophageal 
ganglia and part of the infra-cesophageal ganglia fill up the space Ph. ; 
stretching over the rest of the infra-cesophageal mass is a transverse 
trabecula, which is very thin ; then comes a space in which is seen 
the rest of the infra-oesophageal mass, and then the posterior part of 
the plastron, ventrally to which lies the commencement of the ventral 
nerve-cord. 

In these forms, in wliich the central nervous system is more 




Fig. 61. — A, Entosternite of Limulus; 
B, Entosternite op Thelyphonus. 

Ph., position of pharynx. 



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144 THE ORIGIN OF VERTEBRATES 

concentrated towards the cephalic end than in Limulus, the whole of 
the concentrated brain-mass is separated from the gut only by this thin 
transverse band of tissue. Judging, then, from the entosternite of 
Thelyphonus, it is not difficult to suppose that a continuation of the 
same growth of the brain-region of the central nervous system would 
cause the entosternite to be separated into two lateral trabecule, 
which would then take up the ventro-lateral position of the two 
trabecules of Ammoccetes. 

On the other hand, it might be that two lateral trabecular, 
similar to those of Thelyphonus and situated on each side of the 
central nervous system, were the original form from which, by the 
addition of transverse fibres running between the gut and nervous 
system, the entosternite of Thelyphonus and of the scorpions, etc., 
was formed. From an extensive consideration of the entosternite in 
different animals, Schimkewitsch has come to the conclusion that this 
latter explanation is the true one. He points out that the lateral 
trabecule can be distinguished from the transverse by their structure, 
being much more cellular and less fibrous, and the cell-cavities more 
rounded, or, as I should express it, the two lateral trabecule are more 
cartilaginous, while the transverse are more fibrous. Schimkewitsch, 
from observations of structure and from embryological investi- 
gations, comes to the conclusion that the entosternite was originally 
composed of two parts — 

1. A transverse muscle corresponding to the adductor muscle of 
the shell of certain crustaceans, such as Nebalia. 

2. A pair of longitudinal mesodermic tendons, which may have 
been formed originally out of a number of segmentally arranged 
mesodermic tendons, and are crossed by the fibrils of the transverse 
muscular bundles. 

These paired tendons of the entosternite he considers to corre- 
spond to the intermuscular tendons, situated lengthways, which are 
found in the ventral longitudinal muscles of most arthropods. 

It is clear from these observations of Schimkewitsch, that the 
essential part of the entosternite consists of two lateral trabecule, 
which were originally tendinous in nature and have become of the 
nature of cartilaginous tissue by the increase of cellular elements in 
the matrix of the tissue : these two trabecular function as supports 
for the attachment of muscles, which are specially attached at 
certain places. At these places transverse fibres belonging to some 



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THE EVIDENCE OF THE SKELETON 1 45 

of the muscular attachments cross between the two longitudinal 
trabecule, and so form the transverse trabecule. 

I entirely agree with Schimkewitsch that the nests of cartilage- 
cells are much more extensive in, and indeed nearly entirely 
confined to, these two lateral trabeculaj in the entosternite of 
Hypoctonus. Kay Lankester describes in the entosternite of Mygale 
peculiar cell-nests strongly resembling those of Hypoctonus, and he 
also states that they are confined to the lateral portions of the 
entosternite. 

From this evidence it is easy to see that that portion of the basi- 
cranial skeleton known as the trabecule may have originated from 
the formation of cartilage in the plastron or entosternite of a palae- 
ostracan animal. Such an hypothesis immediately suggests valuable 
clues as to the origin of the cranium and of the rest of the basi- 
cranial skeleton — the parachordals and the auditory capsules. The 
former would naturally be a dorsal extension of the more membranous 
portion of the plastron, in which, equally naturally, cartilaginous tissue 
would subsequently develop ; and the reason why it is impossible to 
reduce the cranium iuto a series of segments would be self-evident, 
for even though, as Schimkewitsch thinks, the plastron may have 
been originally segmented, it has long lost all sign of segmentation. 
The latter would be derived from a second entosternite of the same 
nature as the plastron, but especially connected with the auditory 
apparatus of the invertebrate ancestor. The following out of these 
two clues will be the subject of a future chapter. 

In our search, then, for a clue to the origin of the skeletal tissues 
of the vertebrate we see again that we are led directly to the palaeos- 
tracan stock on the invertebrate side and to the Cyclostomata on that 
of the vertebrate; for in Limulus, the only living representative of 
the Pakeostraca, and in Limulus alone, we find a skeleton marvel- 
lously similar to the earliest vertebrate skeleton— that found in 
Ainmocoetes. Later on I shall give reasons for the belief that the 
earliest fishes so far found, the Cephalaspidee, etc., were built up on 
the same plan as Ammoccetes, so that, in my opinion, in Limulus 
and in Ammoccetes we actually possess living examples allied to 
the ancient fauna of the Silurian times. 



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146 THE ORIGIN OF VERTEBRATES 



SUMMAltY. 

The skeleton considered in this chapter is not the notochord, but that 
composed of cartilage. The tracing downwards of the vertebrate bony and 
cartilaginous skeleton to its earliest beginnings leads straight to the skeleton of 
the larval lamprey (Ammoccetes), in which vertebras are not yet formed, but the 
cranial and branchial skeleton is well marked. 

The embryological and phylogenetic histories are in complete unison to show 
that the cranial skeleton is older than the spinal, and this primitive branchial 
skeleton is also in harmony with the laws of evolution, in that its structure, even 
in the adult lamprey (Petromyzon), never gets beyond the stage characteristic 
of embryonic cartilage in the higher vertebrates. 

The simplest and most primitive skeleton is that found in Ammocoetes and 
consists of two parts : (1) a prosomatic, (2) a mesosomatic skeleton. 

The prosomatic skeleton forms a non-segmented basi-cranial skeleton of the 
simplest kind— the trabecule and the parachordals with their attached auditory 
capsules, just as the embryology of the higher vertebrates teaches us must be 
the case. There in the free-living, still-existent Ammocoetes we find the manifest 
natural outcome of the embryological history in the shape of simple trabecular 
and parachordals, from which the whole complicated basi-cranial skeleton of the 
higher vertebrates arose. 

The mesosomatic skeleton, which is formed before the prosomatic, consisted* 
in the first instance, of simple branchial bars segmentally arranged, which were 
connected together by a longitudinal subchordal bar, situated laterally on each 
side of the notochord. These simple branchial bars later on form the branchial 
basket-work, which forms an open-work cage within which the branchiae are 
situated. 

The cartilages which compose these two skeletons respectively are markedly 
different in chemical constitution, in that the first (hard cartilage) is mainly 
composed of chondro-gelatin, the second (soft cartilage) of chondro-mucoid 
material. 

The same kind of difference is seen in the two kinds of connective tissue 
which are the forerunners of these two kinds of cartilage. Thus, the cranial 
walls in Ammocoetes are formed of white fibrous tissue, an essentially gelatin- 
containing tissue ; at transformation these are invaded by chondro-blasts and 
the cartilaginous cranium, formed of hard cartilage, results. On the other hand, 
the forerunner of the branchial soft cartilage is a very striking and peculiar 
kind of connective tissue loaded with mucoid material, to which the name 
muco-cartilage has been given. 

The enormous interest of this muco-cartilage consists in the fact that it 
forms very well-defined plates of tissue, entirely confined to the head-region, 
which are not found in any higher vertebrate, not even in the adult form 
Petromyzon, for every scrap of the tissue as such disappears at transformation. 

It is this evidence of primitive non-vertebrate tissues, which occur in the 
larval but not in the adult form, which makes Ammocoetes so valuable for the 
investigation of the origin of vertebrates. 

The evidence, then, is extraordinarily clear as to the beginnings of the 
vertebrate skeletal tissues. 



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THE EVIDENCE OF THE SKELETON 1 47 

In the invertebrate kingdom true cartilage occurs but scantily. There is 
a cartilaginous covering of the brain of cephalopoda. It is never found in crabs, 
lobsters, bees, wasps, centipedes, butterflies, flies, or any of the great group of 
Arthropoda, except, to a slight extent, in some members of the scorpion group* 
and more fully in one single animal, the King-crab or Limulus : a fact significant 
of itself, but still more so when the nature of the cartilage and its position in 
the animal is taken into consideration, for the identity both in structure and 
position of this internal cartilaginous skeleton with that of Ammocoetes is 
extraordinarily great. 

Here, in Limulus, just as in Ammocoetes, an internal cartilaginous skeleton 
is found, composed of two distinct parts : (1) prosomatic, (2) mesosomatic. As 
in Ammocoetes, the latter consists of simple branchial bars, segmentally arranged, 
which are connected together on each side by a longitudinal ligament contain- 
ing cartilage — the entapophysial ligament. This cartilage is identical in 
structure and in chemical composition with the soft cartilage of Ammocoetes, 
and, as in the latter case, arises in a markedly mucoid connective tissue. 
The former, as in Ammocoetes, consists of a non-segmental skeleton, the 
plastron, composed of a white fibrous connective tissue matrix, an essentially 
gelatin-containing tissue, in which are found nests of cartilage cells of the 
hard cartilage variety. 

This remarkable discovery of the branchial cartilaginous bars of Limulus, 
together with that of the internal prosomatic plastron, causes the original diffi- 
culty of deriving an animal such as the vertebrate from an animal resembling 
an arthropod to vanish into thin air, for it shows that in the past ages when the 
vertebrates first appeared on the earth, the dominant arthropod race at that time, 
the members of which resembled Limulus, had solved the question ; for, in addition 
to their external chitinous covering, they had manufactured an internal cartila- 
ginous skeleton. Not only so, but that skeleton had arrived, both in structure 
and position, exactly at the stage at which the vertebrate skeleton starts. 

What the precise steps are by which chi tin- formation gives place to chondrin- 
formation are not yet fully known, but Schmiedeberg has shown that a substance, 
glycosamine, is derivable from both these skeletal tissues, and he concludes his 
observations in the following words: u Thus, by means of glycosamine, the 
bridge is formed which connects together the chitin of the lower animals with 
the cartilage of the more highly organized creations." 

The evidence of the origin of the cartilaginous skeleton of the vertebrate 
points directly to the origin of the vertebrate from the Pakeostraca, and is 
of so strong a character that, taken alone, it may almost be considered as proof 
of such origin. 



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CHAPTER IV 

THE EVIDENCE OF THE RESPIRATORY APPARATUS 

Branchiae considered as internal branchial appendages. — Innervation of branchial 
segments. — Cranial region older than spinal. — Three-root system of cranial 
nerves, dorsal, lateral, ventral. — Explanation of van Wijhe's segments. — 
Lateral mixed root is appendage-nerve of invertebrate. — The branchial 
chamber of Ammoccetes. — The branchial unit, not a pouch but an 
appendage. — The origin of the branchial musculature. — The branchial 
circulation. — The branchial heart of the vertebrate. — Not homologous with 
the systemic heart of the arthropod. — Its formation from two longitudinal 
venous sinuses. — Summary. 

The respiratory apparatus in all the terrestrial vertebrates is of the 
same kind — one single pair of lungs. These lungs originate as a 
diverticulum of the alimentary canal. On the other hand, the 
aquatic vertebrates breathe by means of a series of branchiae, or gills, 
which are arranged segmentally, being supported by the segmental 
branchial cartilaginous bars, as already mentioned in the last chapter. 
The transition from the gill-bearing to the lung-bearing vertebrates 
is most interesting, for it has been proved that the lungs are formed 
by the modification of the swim-bladder of fishes ; and in a group 
of fishes, the Dipnoi, or lung-fishes, of which three representatives 
still exist on the earth, the mode of transition from the fish to the 
amphibian is plainly visible, for they possess both lungs and 
gills, and yet are not amphibians, but true fishes. But for the 
fortunate existence of Ceratodus in Australia, Lepidosiren in South 
America, and Protopterus in Africa, it would have been impossible 
from the fossil remains to have asserted that any fish had ever 
existed which possessed at the same moment of time the two kinds 
of respiratory organs, although from our knowledge of the develop- 
ment of the amphibian we might have felt sure that such a transitional 
stage must have existed. Unfortunately, there is at present no 
likelihood of any corresponding transitional stage being discovered 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 49 

living on the earth in which both the dorsal arthropod alimentary 
canal and the ventral vertebrate one should simultaneously exist in 
a functional condition ; still it seems to me that even if Ceratodus, 
Lepidosiren, and Protopterus had ceased to exist on the earth, yet 
the facts of comparative anatomy, together with our conception of 
evolution as portrayed in the theory of natural selection, would have 
forced us to conclude rightly that the amphibian stage in the evolu- 
tion of the vertebrate phylum was preceded by fishes which possessed 
simultaneously lungs and gills. 

In the preceding chapter the primitive cartilaginous vertebrate 
skeleton, as found in Ammocoetes, was shown to correspond in a 
marvellous manner to the cartilaginous skeleton of Limulus. In a 
later chapter I will deal with the formation of the cranium from the 
prosomatic skeleton ; in this chapter it is the mesosomatic skeleton 
which is of interest, and the consideration of the necessary conse- 
quences which logically follow upon the supposition that the branchial 
cartilaginous bars of Limulus are homologous with the branchial 
basket-work of Ammocoetes. 

Internal Branchial Appendages. 

Seeing that in both cases the cartilaginous bars of Limulus and 
Ammoccrtes are confined to the branchial region, their homology of 
necessity implies an homology of the two branchial regions, and leads 
directly to the conclusion that the branchiae of the vertebrate were 
derived from the branchiae of the arthropod, a conclusion which, 
according to the generally accepted view of the origin of the respira- 
tory region in the vertebrate, is extremely difficult to accept ; for the 
branchiae of Limulus and of the Arthropoda in general are part of 
the mesosomatic appendages, while the branchiae of vertebrates are 
derived from the anterior part of the alimentary canal. This con- 
clusion, therefore, implies that the vertebrate has utilized in the 
formation of the anterior portion of its new alimentary canal the 
branchial appendages of the palaeostracan ancestor. 

Let us consider dispassionately whether such a suggestion is a priori 
so impossible as it at first appears. One of the principles of evolution 
is that any change which is supposed to have taken place in the 
process of formation of one animal or group of animals from a lower 
group must be in harmony with changes which are known to have 



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'5° 



THE ORIGTN OF VERTEBRATES 



occurred in that lower group. On the assumption, therefore, that 
the vertebrate branchiae represent the branchial portion of the 
arthropod mesosomatic appendages which have sunk in and so 
become internal, we ought to find that in members of this very 
group such inclusion of branchial appendages has taken place. This, 
indeed, is exactly what we do find, for in all the scorpion tribe, which 
is acknowledged to be closely related to Limulus, there are no 

external mesosomatic appendages, 
but in all cases these appendages 
have sunk into the body, have 
disappeared as such, and retained 
only the vital part of them — the 
branchiae. In this way the so-called 
lung - books of the scorpion are 
formed, which are in all respects 
homologous with the branchiae or 
gill-books of Limulus. Now, as 
already mentioned, the lords of 
creation in the palaeostracan times 
were the sea-scorpions, which, as is 
seen in Fig. 62, resembled the land- 
scorpions of the present day in the 
entire absence of any external ap- 
pendages on the segments of the 
mesosomatic region. As they lived 
in the sea, they must have breathed 
with gills, and those branchial ap- 
pendages must have been internal, 
just as in the land-scorpions of the 
present time. Indeed, markings 
have been found on the internal 
side of the segments 1-5, Fig. 62, which are supposed to indicate 
branchue, and these segments are therefore supposed to have borne 
the branchiae. Up to the present time no indication of gill-slits 
has been found, and we cannot say with certainty how these 
animals breathed. Further, in the Upper Silurian of Lesmahago, 
Lanarkshire, a scorpion (Palccophonus Hunteri), closely resembling the 
modern scorpion, has been found, which, as Lankester states, was in 
all probability aquatic, and not terrestrial in its habits. How it 




Fig. 62. — Eurypterus. 

The segments and appendages on the 
right are numbered in correspon- 
dence with the cranial system of 
lateral nerve-roots as found in verte- 
brates. M. y me tastoma. The surface 
ornamentation is represented on 
the first segment posterior to the 
branchial segments. The opercular 
appendage is marked out by dots. 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS 151 

breathed is unknown ; it shows no signs of stigmata, such as exist in 
the scorpion of to-day. 

Although we possess as yet no certain knowledge of the position 
of the gill-openings in these ancient scorpion-like forms, what we 
can say with certainty — and that is the important fact — is, that at 
the time when the vertebrates appeared, a very large number of the 
dominant arthropod race possessed internally-situated branchiae, which 
had been directly derived from the branchiae-bearing appendages of 
their Limulus-like kinsfolk. 

This abolition of the branchiae-bearing appendages as external 
organs of locomotion, with the retention of the important branchial 
portion of the appendage as internal branchiae, is a very important 
suggestion in any discussion of the way vertebrates have arisen from 
arthropods; for, if the same principle is of universal application, it 
leads directly to the conclusion that whenever an appendage possesses 
an organ of vital importance to the animal, that organ will remain, 
even though the appendage as such completely vanishes. Thus, as 
will be shown later, special sense-organs such as the olfactory remain, 
though the animal no longer possesses antennae ; the important ex- 
cretory organs, the coxal glands, and important respiratory organs, 
the branchiae, are still present in the vertebrate, although the appen- 
dages to which they originally belonged have dwindled away, or, at 
all events, are no longer recognizable as arthropod appendages. 

Innervation of Branchial Segments. 

Passing from a priori considerations to actual facts, it is advisable 
to commence with the innervation of the branchial segments; for, 
seeing that the foundation of the whole of this comparative study 
of the vertebrate and the arthropod is based upon the similarity of 
the two central nervous systems, it follows that we must look in 
the first instance to the innervation of any organ or group of organs 
in order to find out their relationship in the two groups of animals. 

The great characteristic of the vertebrate branchial organs is their 
segmental arrangement and their innervation by the vagus group of 
nerves, i.e. by the hindermost group of the cranial segmental nerves. 
These cranial nerves are divided by Gegenbaur into two great groups 
— an anterior group, the trigeminal, which supplies the muscles of 
mastication, and a posterior group, the vagus, which is essentially 



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152 THE ORIGIN OF VERTEBRATES 

respiratory in function. Of these two groups, I will consider the 
latter group first. 

In Limulus the great characteristic of the branchial region is its 
uronounced segmental arrangement, each pair of branchial appendages 
belonging to a separate segment. This group of segments forms the 
mesosoma, and these branchial appendages are the mesosomatic 
appendages. Anterior to them are the segments of the prosoma, 
which bear the prosomatic or locomotor appendages. The latter are 
provided at their base with gnathites or masticating apparatus, so 
that the prosomatic group of nerves, like the trigeminal group in the 
vertebrate, comprises essentially the nerves subserving the important 
function of mastication. As already pointed out, the brain-region 
of the vertebrate is comparable to the supra-oesophageal and infra- 
cesophageal ganglia of the invertebrate, and it has been shown (p. 54) 
how, by a process of concentration and cephalization, the foremost 
region of the infra-ccsophageal ganglia becomes the prosomatic region, 
and is directly comparable to the trigeminal region in the vertebrate ; 
while the hindermost region is formed from the concentration of 
the mesosomatic ganglia, and is directly comparable to the medulla 
oblongata, i.e. to the vagus region of the vertebrate brain. 

As far, then, as concerns the centres of origin of these two groups 
of nerves and their exits from the central nervous system, they are 
markedly homologous in the two groups of animals. 

Comparison of the Cranial and Spinal Segmental Nerves. 

It has often been held that the arrangements of the vertebrate 
nervous system differ from those of other segmented animals in one 
important particular. The characteristic of the vertebrate is the 
origin of every segmental nerve from two roots, of which one con- 
tains the efferent fibres, while the other possesses a sensory ganglion, 
and contains only afferent fibres. This arrangement, which is found 
along the whole spinal cord of all vertebrates, is not found in the 
segmental nerves of the invertebrates ; and as it is supposed that the 
simpler arrangement of the spinal cord was the primitive arrange- 
ment froin which the vertebrate central nervous system was built up, 
it is often concluded that the animal from which the vertebrate arose 
must have possessed a series of nerve segments, from each of which 
there arose bilaterally ventral (efferent) and dorsal (afferent) roots. 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 53 

Now, the striking fact of the vertebrate segmental nerves consists 
in this, that, as far as their structure and the tissues which they 
innervate are concerned, the cranial segmental nerves are built up on 
the same plan as the spinal ; but as far as concerns their exit from 
the central nervous system they are markedly different. A large 
amount of ingenuity, it is true, has been spent in the endeavour to 
force the cranial nerves into a series of segmental nerves, which 
arise in the same way as the spinal by two roots, of which the ven- 
tral series ought to be efferent and the dorsal series afferent, but 
without success. We must, therefore, consider the arrangement of 
the cranial segmental nerves by itself, separately from that of the 
spinal nerves, and the problem of the origin of the vertebrate seg- 
mental nerves admits of two solutions — either the cranial arrange- 
ment has arisen from a modification of the spinal, or the spinal from 
a simplification of the cranial. The first solution implies that the 
spinal cord arrangement is older than the cranial, the second that 
the cranial is the oldest. 

In my opinion, the evidence of the greater antiquity of the cranial 
region is overwhelming. 

The evidence of embryology points directly to the greater phylo- 
genetic antiquity of the cranial region, for we see how, quite early in 
the development, the head is folded off, and the organs in that 
region thereby completed at a time when the spinal region is only at 
an early stage of development. We see how the first of the trunk 
somites is formed just posteriorly to the head region, and then more 
and more somites are formed by the addition of fresh segments poste- 
riorly to the one first formed. We see how, in Ammocoetes, the first 
formed parts of the skeleton are the branchial bars and the basi- 
cranial system, while the rudiments of the vertebrae do not appear 
until the Petromyzon stage. We see how, with the elongation of the 
animal by the later addition of more and more spinal segments, 
organs, such as the heart, which were originally in the head, travel 
down, and the vagus and lateral-line nerves reach their ultimate 
destination. Again, we see that, whereas the cranial nerves, viz. the 
ocular motor, the trigeminal, facial, auditory, glossopharyngeal, and 
vagus nerves, are wonderfully fixed and constant in all vertebrates, 
the only shifting being in the spino-occipital region, in fact, at the 
junction of the cranial and spinal region, the spinal nerves, on the 
other hand, are not only remarkably variable in number in different 



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154 THE ORIGIN OF VERTEBRATES 

groups of animals, but that even in the same animal great variations 
are found, especially in the manner of formation of the limb-plexuses. 
Such marked meristic variation in the spinal nerves, in contrast to 
the fixed character of the cranial nerves, certainly points to a more 
recent formation of the former nerves. 

Also the observations of Assheton on the primitive streak of the 
rabbit, and on the growth in length of the frog embryo, have led 
him to the conclusion that, as in the rabbit so in the frog, there 
is evidence to show that the embryo is derived from two definite 
centres of growth : the first, phylogenetically the oldest, being a 
protoplasmic activity, which gives rise to the anterior end of the 
embryo ; the second, one which gives rise to the growth in length of 
the embryo. This secondary area of proliferation coincides with the 
area of the primitive streak, and he has shown, in a subsequent 
paper, by means of the insertion of sable hairs into the unincubated 
blastoderm of the chick, that a hair inserted into the centre of the 
blastoderm appears at the anterior end of the primitive streak, and 
subsequently is found at the level of the most anterior pair of somites. 

He then goes on to say — 

"From these specimens it seems clear that all those parts in 
front of the first pair of mesoblastic somites — that is to say, the 
heart, the brain and medulla oblongata, the olfactory, optic, auditory 
organs and foregut — are developed from that portion of the un- 
incubated blastoderm which lies anterior to the centre of the blasto- 
derm, and that all the rest of the embryo is formed by the activity 
of the primitive streak area." 

In other words, the secondary area of growth, i.e. the primitive 
streak area, includes the whole of the spinal cord region, while the 
older primary centre of growth is coincident with the cranial region. 

In searching, then, for the origin of the segmental nerves, we 
must consider the type on which the cranial nerves are arranged 
rather than that of the spinal nerves. 

The first striking fact occurs at the spino-occipital region, where 
the spinal cord merges into the medulla oblongata, for here in the 
cervical region we find each spinal segment gives origin to three dis- 
tinct roots, not two — a dorsal root, a ventral root, and a lateral root. 
This third root gives origin to the spinal accessory nerve, and in the 
region of the medulla oblongata these lateral roots merge directly 
into the roots of the vagus nerve ; more anteriorly the same system 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 55 

continues as the roots of the glossopharyngeal nerve, as the roots of 
the facial nerve, and as a portion, especially the motor portion, of 
the trigeminal nerve. Now, all these nerves belong to a well-defined 
system of nerves, as Charles Bell l pointed out in 1830, a system of 
nerves concerned with respiration and allied mechanisms, such as 
laughing, sneezing, mastication, deglutition, etc., nerves innervating a 
set of muscles of very different kind from the ordinary body-muscles 
concerned with locomotion and equilibration. Also the centres from 
which these motor nerves arise are well defined, and form cell-masses 
in the central nervous system, quite separate from those which give 
origin to somatic muscles. 

This original idea of Charles Bell, after having been ignored for so 
long a time, is now seen to be a very right one, and it is an extra- 
ordinary thing that his enunciation of the dual nature of the spinal 
roots, which was, to his mind, of subordinate importance, should so 
entirely have overshadowed his suggestion, that in addition to the 
dorsal and ventral roots, a lateral system of nerves existed, which 
were not exclusively sensory or exclusively motor, but formed a 
separate system of respiratory nerves. 

Further, anatomists divide the striated muscles of the body into 
two great natural groups, characterized by a difference of origin and 
largely by a difference of appearance. The one set is concerned 
with the movements of internal organs, and is called visceral, the 
other is derived from the longitudinal sheet of musculature which 
forms the myotomes of the fish, and has been called parietal or 
somatic. The motor nerves of these two sets of muscles correspond 
with the lateral or respiratory and ventral roots respectively. 

Finally, it has been shown that the segments of which a verte- 
brate is composed are recognizable in the embryo by the segmented 
manner in which the musculature is laid down, and van Wijhe has 
shown that in the cranial region two sets of muscles are laid down 
segmentally, thus forming a dorsal and ventral series of commencing 
muscular segments. Of these the anterior segments of the dorsal 
series give origin to the striated muscles of the eye which are inner- 
vated by the Illrd (oculomotor), IVth (trochlearis), and Vlth (ab- 
ducens) nerves, while the posterior segments give origin to the 

1 N.B. — In addition to the nerves mentioned, C. Bell included, in his respiratory 
system of nerves, the fourth nerve or trochlearis, the phrenic and the external 
respiratory of Bell. 



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156 THE ORIGIN OF VERTEBRATES 

muscles from the cranium to the shoulder-girdle, innervated by the 
Xllth (hypoglossal) nerve. The ventral series of segments give 
origin to the musculature supplied by the trigeminal, facial, glosso- 
pharyngeal, and vagus nerves. 

Also, the afferent or sensory nerves of the skin over the whole of 
this head-region are supplied by the trigeminal nerve, while the 
afferent nerves to the visceral surfaces are supplied by the vagus, 
glossopharyngeal and facial nerves. 

In van Wijhe's original paper he arranged the segments belonging 
to the cranial nerves in the following table : — 



Segment*. 


Ventral nerve-roots and muscles 
derived from myotomes. 


Visceral clefts. 


Dorsal nerv 
V. N.op- 


e-roots and muscles. 


1 


III. 


' M. rectus supe- 










rior, m. rectus 


thalmicus 








intemus, m. 


profundus 








rectus inferior, 










m. obliquus in- I 










ferior 






2 


IV. 


M. obliquus 
| superior 1st Mandibular 


V. 


Masticating 
muscles. 


3 
4 


VI. 


M. rectus ex- , 
ternus /tj-t^:^ 
\Hyoid, 


VII., 
VII., 


(Facial muscles 
{ (VIII. is dorsal 
( branch of VII.) 


5 


— 


— 3rd 1st Branchial 


IX. 


) 


G 

7 
8 


XII. 
XII. 


— 4th 2nd 
| Muscles from ( 5th 3rd „ 
> cranium to < 6th 4th „ 


x. 3 


f 

Branchial and 
visceral muscles 


9 


XII. 


shoulder-girdle 1 

1 


7th 5th 


x., 


1 



As is seen in the table, van Wijhe attempts to arrange the cranial 
segmental nerves into dorsal and ventral roots, in accordance with 
the arrangement in the spinal region. In order to do this he calls 
the Vth, Vllth, IXth, and Xth nerves dorsal roots, although they 
are not purely sensory nerves, but contain motor fibres as well. 

It is not accidental that he should have picked out for his dorsal 
roots the very nerves which form Charles Bell's lateral series of 
roots, inasmuch as this system of lateral roots, apart from dorsal and 
ventral roots, really is, as Charles Bell thought, an important separate 
system, dependent upon a separate segmentation in the embryo of 
the musculature supplied by these roots. This segmentation may 
receive the name of visceral or splanchnic in contradistinction to 
somatic, since all the muscles without exception belong to the visceral 
group of striated muscles. 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 57 

These observations of van Wijhe lead directly to the following 
conclusion. In the cranial region there is evidence of a double set 
of segments, which may be called somatic and splanchnic. The 
somatic segments, consisting of the outer skin and the body muscu- 
lature, are doubly innervated as are those of the spinal cord by a 
series of ventral motor roots, the oculomotor or lllrd nerve, the 
trochlear or IVth nerve, the abducens or Vlth nerve, and the hypo- 
glossal or Xllth nerve, and by a series of dorsal sensory roots, the 
sensory part of the trigeminal or Vth nerve. But the splanchnic 
segments are innervated by single roots, the vagus or Xth nerve, 
glossopharyngeal or IXth nerve, facial or Vllth nerve, and trigeminal 
or Vth nerve, which are mixed, containing both sensory and motor 
fibres, thus differing markedly from the arrangement of the spinal 
nerves. 

From this sketch it follows that the arrangement seen in the 
spinal cord, would result from the cranial arrangement if this third 
system of lateral roots were left out. Further, since the cranial 
system is the oldest, we must search in the invertebrate ancestor for 
a tripartite rather than a dual system of nerve-roots for each segment ; 
a system composed of a dorsal root supplying only the sensory nerves 
of the skin-surfaces, a lateral mixed root supplying the system con- 
nected with respiration with both sensory and motor fibres, and a 
ventral root supplying the motor nerves to the body-musculature. 

Comparison of the Appendage Nerves of Limulus and Branchi- 
pus to the Lateral Root System of the Vertebrate. 

If the argument used so far is correct, and this tripartite system 
of nerve-roots, as seen in the cranial nerves of the vertebrate, really 
represents the original scheme of innervation in the palaeostracan 
ancestor, then it follows that each segment of Limulus ought to be 
supplied by three nerves— (1), a sensory nerve supplying its own 
portion of the skin-surface of the prosomatic and mesosomatic 
carapaces; (2), a lateral mixed nerve supplying exclusively the 
appendage of the segment, for the appendages carry the respiratory 
organs; and (3), a motor nerve supplying the body-muscles of the 
segment. 

It is a striking fact that Milne-Edwards describes the nerve-roots 
in exactly this manner. The great characteristic of the nerve-roots 



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158 THE ORIGIN OF VERTEBRATES 

in Liinulus as in other arthropods is the large appendage-nerve, 
which is always a mixed nerve ; in addition, there is a system of 
sensory nerves to the prosomatic and mesosomatic carapaces, called 
by him the epimeral nerves, which are purely sensory, and a third 
set of roots which are motor to the body-muscles, and possibly also 
sensory to the ventral surface between the appendages. 

Moreover, just as in the vertebrate central nervous system the 
centres of origin of the motor nerves of the branchial segmentation 
are distinct from those of the somatic segmentation, so we find, from 
the researches of Hardy, that a similar well-marked separation exists 
between the centres of origin of the motor nerves of the appendages 
and those of the somatic muscles in the central nervous system of 
Branchipus and Astacus. 

In the first place, he points out that the nervous system of 
Branchipus is of a very primitive arthropod type ; that it is, in fact, 
as good an example of an ancient type as we are likely to find in the 
present day ; a matter of some importance in connection with my 
argument, since the arthropod ancestor of the vertebrate, such as I 
am deducing from the study of Ammoccetes, must undoubtedly have 
been of an ancient type, more nearly connected with the strange 
forms of the trilobite era than with the crabs and spiders of the 
present day. 

His conclusions with respect to Branchipus may be tabulated as 
follows : — 

1. Each ganglion of the ventral chain is formed mainly for the 
innervation of the appendages. 

2. Each ganglion is divided into an anterior and posterior division, 
which are connected respectively with the motor and sensory nerves 
of the appendages. 

3. The motor nerves of the appendages arise as well-defined axis- 
cylinder processes of nerve-cells, which are arranged in well-defined 
groups in the anterior division of the ganglion. 

4. A separate innervation exists for the muscles and sensory 
surfaces of the trunk. The trunk-muscles consist of long bundles, 
from which slips pass off to the skin in each segment ; they are thus 
imperfectly segmented. In accordance with this, a difluse system 
of nerve-fibres passes to them from certain cells on the dorsal surface 
of each lateral half of the ganglion. These cell-groups are therefore 
very distinct from those which give origin to the motor appendage- 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 59 

nerves, and, moreover, are not confined to the ganglion, but extend 
for some distance into the interganglionic region of the nerve-cords 
which connect together the ganglia of the ventral chain. 

Hardy's observations, therefore, combined with those of Milne- 
Edwards, lead to the conclusion that in such a primitive arthropod 
type as my theory postulates, each segment was supplied with 
separate sensory and motor somatic nerves, and with a pair of nerves 
of mixed function, devoted entirely to the innervation of the pair of 
appendages ; that also, in the central nervous system, the motor 
nerve-centres were arranged in accordance with a double set of seg- 
mented muscles in two separate groups of nerve-cells. These nerve- 
cells in the one case were aggregated into well-defined groups, which 
formed the centres for the motor nerves of the markedly segmented 
muscles of the appendages, and in the other case formed a system of 
more diffused cells, less markedly aggregated into distinct groups, 
which formed the centres for the imperfectly segmented somatic 
muscles. 

Such an arrangement suggests that in the ancient arthropod type 
a double segmentation existed, viz. a segmentation of the body, and 
a segmentation due to the appendages. Undoubtedly, the segments 
originally corresponded absolutely as in Branchipus, and every 
appendage was attached to a well-defined separate body-segment. 
In, however, such an ancient type as Limulus, though the segmen- 
tation may be spoken of as twofold, yet the number of segments 
in the prosomatic and mesosomatic regions are much more clearly 
marked out by the appendages than by the divisions of the soma ; 
for, in the prosomatic region such a fusion of somatic segments 
to form the tergal prosomatic carapace has taken place that the 
segments of which it is composed are visible only in the young con- 
dition, while in the mesosomatic region the separate somatic segments, 
though fused to form the mesosomatic carapace, are still indicated 
by the entapophysial indentations. 

Clearly, then, if the mesosomatic branchial appendages of forms 
related to Limulus were reduced to the branchial portion of the 
appendage, and that branchial portion became internal, just as is 
known to be the case in the scorpion group, we should obtain an 
animal in which the mesosomatic region would be characterized by 
a segmentation predominantly branchial, which might be termed, as 
in vertebrates, the branchiovicric segmentation, but yet would show 



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160 THE ORIGIN OF VERTEBRATES 

indications of a corresponding somatic or mesotncric segmentation. 
The nerve supply to these segments would consist of — 

1. The epiineral purely sensory nerves to the somatic surface, 
equivalent in the vertebrate to the ascending root of the trigeminal. 

2. The mixed nerves to the internal branchial segments, equivalent 
in the vertebrate to the vagus, glossopharyngeal, and facial. 

3. The motor nerves to the somatic muscles, equivalent in the 
vertebrate to the original nerve-supply to the somatic muscles 
belonging to these segments, i.e. to the muscles derived from van 
Wijhe's 4th, 5th, and 6th somites. 

Further, the centres of origin of these appendage-nerves would 
form centres in the central nervous system separate from the centres 
of the motor nerves to the somatic muscles, just as the centres of 
origin of the motor parts of the facial, vagus, and glossopharyngeal 
nerves form groups of cells quite distinct from the centres for the 
hypoglossal, abducens, trochlear, and oculomotor nerves. 

In fact, if the vertebrate branchial nerves are looked upon as the 
descendants of nerves which originally supplied branchial appendages, 
then every question connected with the branchial segmentation, with 
the origin and distribution of those nerves, receives a simple and 
adequate solution — a solution in exact agreement with the conclusion 
that the vertebrate arose from a ] uiheostracan ancestor. 

It would, therefore, be natural to expect that the earliest fishes 
breathed by means of branchial appendages situated internally, and 
that the evidence for such appendages would be much stronger in 
them than in more recent fishes. 

Although we know nothing of the nature of the respiratory appa- 
ratus in the extinct fishes of Silurian times, we have still living, in 
the shape of Ammocootes, a possible representative of such types. 
If, then, we find, as is the case, that the respiratory apparatus of 
Amuioccutes differs markedly from that of the rest of the fishes, and, 

sed, bom that of the adult form or Petromyzon, and that that 

- difi'ereuce consists in a givau-r resemblance to internal branchial 
appendages in the case of Ainnioctetes, then we may feel that the 
of the origin of the branchial apparatus of the vertebrate from 
ternal branchial appendages of the invertebrate has gained 

mously. 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS l6l 

The Respiratory Chamber of Ammoccetes. 

In order to make clear the nature of the branchial segments in 
Ammocoetes, I have divided the head-part of the animal by means of 
a longitudinal horizontal section into halves — ventral and dorsal — 
as shown in Figs. 63 and 64. These figures are each a combination 
of a section and a solid drawing. The animal was slit open by a 
longitudinal section in the neighbourhood of the gill-slits, and each 
half was slightly flattened out, so as to expose the ventral and dorsal 
internal surfaces respectively. The structures in the cut surface were 
drawn from one of a series of horizontal longitudinal sections taken 
through the head of the animal These figures show that the head-region 
of Ammocoetes consists of two chambers, the contents of which are 
different. In front, an oral or stomodaeal chamber, which contains the 
velum and tentacles, is enclosed by the upper and lower lips, and was 
originally separated by a septum from the larger respiratory chamber, 
which contains the separate pairs of branchiae. A glance at the two 
drawings shows clearly that Rathke's original description of this 
chamber is the natural one, for he at that time, looking upon Ammo- 
eMes branchialis as a separate species, described the branchial chamber 
as containing a series of paired gills, with the gill-openings between 
consecutive gills. His branchial unit or gill, therefore, was repre- 
sented by each of the so-called diaphragms, which, as seen in Figs. 63, 
64, are all exactly alike, except the first and the last. Any one of 
these is represented in section in Fig. 65, and represents a branchial 
unit in Rathke's view and in mine. Clearly, it may be described as a 
branchial appendage which projects into an open pharyngeal chamber, 
so that the series of such appendages divides the chamber into a 
series of compartments, each of which communicates with the exterior 
by means of a gill-slit, and with each other by means of the open 
space between opposing appendages. 

Each of these appendages possesses its own cartilaginous bar 
(Br. cart.), as explained in Chapter III. ; each possesses its own bran- 
chial or visceral muscles (coloured blue in Figs. 63 and 64), separated 
absolutely from the longitudinal somatic muscles (coloured dark 
red in Figs. 63 and 64) by a space (Sp.) containing blood and 
peculiar fat-cells, etc. Each possesses its own afferent branchial 
blood-vessel from the ventral aorta, and its own efferent vessel to 
the dorsal aorta (Fig. 65, a. b)\ and v. Jr.). Each possesses its own 

M 



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Respiratory Afpen 
« Herve Supply 



Huoid. 
Vf[ 



1*-' Br 
IX 




X 



TPiqment 



Fio. G3. — Ventral h 
ok Head-region of . 
moccei'es. 



Somatic implies coloured 
red. Branchial and visce- 
ral muscles coloured blue. 
Tubular constrictor mus- 
cles distinguished from 
striated constrictor mus- 
cles by simple batching. 
Tent., tentacles ; Tent, m.r., 
muco-cartilage of tenta- 
cles; Vel, m.c. muco-car- 
tilage of the velum ; Hy. 
m.c, muco-cartilage of the 
hyoid segment; Px. or., 
pseudo-branchial groove ; 
Br. cart.i branchial carti- 
lages ; 8p. t space between 
somatic and splanchnic 
muscles ; 7A. <>/>., orifice of 
thyroid ; //., heart. 



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Tr -^ 



Ser. — 



Fig. 64. — Dorsal 

HALF Ol 111: \ I ► - 
REGION OF Am- 
MOCOETR! 




Inf. 



7V., trabeculae; 
Pit., pituitary 
space; /«/*., in- 
t u iid ibulum ; 
median ser- 
rated flange of 
velar folds. 



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164 



THE ORIGIN OF VERTEBRATES 



segmental nerve, which supplies its own branchial muscles and no 
others with motor fibres, and sends sensory fibres to the general surface 
of each appendage, as also to the special sense-organs in the shape 
of the epithelial pits (&, Fig. 65) arranged along the free edges of 




"ci.v 



Fig. 65. — Section through Branchial Ap- 

NDAGK OF AMMOCCETES. 

br. cart., branchial cartilage; v. br., branchial 
iu; a. br. f branchial artery; 6.S., blood- 
spaces ; p., pigment ; S., sense-organ ; c, cili- 
ated band; E., J., external and internal 
borders; m. add., adductor muscle; m 
striated constrictor muscle; m.c.t., tubular 
tor muscle; m. and rn.r., muscles 



br.cart. 



Fig. 66.— Section through Bran- 
chial Appendage op Limulus. 

br. cart., branchial cartilage; 
v.&r., branchial vein ; b.s., blood- 
spaces formed by branchial artery ; 
P., pigment; m u posterior enta- 
pophysio-branchial muscle ; m,, 
anterior entapophysio-branchial 
muscle; m if external branchial 
muscle. 



3 nerves possesses its own ganglion — 
lion. 

1 shown that the segmental branchial 
utely such an appendage or branchial 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 65 

segment, and does not supply any portion of the neighbouring branchial 
segments. The nerve-supply in Ammocoetes gives no countenance to 
the view that the original unit was a branchial pouch, the two sides 
of which each nerve supplied, but is strong evidence that the original 
unit was a branchial appendage, which was supplied by a single 
nerve with both motor and sensory fibres. 

Any observer having before him only this picture of the respiratory 
chamber of Ammocoetes, upon which to base his view of a vertebrate 
respiratory chamber, would naturally look upon the branchial unit of 
a vertebrate as a gilled appendage projecting into the open cavity of 
the anterior part of the alimentary canal or pharynx. This is not, 
however, the usual conception. The branchial unit is ordinarily 
described as a gill-pouch, which possesses two openings or slits, an 
internal one into the lumen of the alimentary canal, and an external 
one into thfe surrounding medium. This view is based upon embryo- 
logical evidence of the following character : — 

The alimentary canal of all vertebrates forms a tube stretching 
the whole length of the animal; the anterior part of this tube 
becomes pouched on each side at regular intervals, and the walls of 
each pouch becoming folded form the respiratory surfaces or gills. 
The openings of these separate pouches into the central lumen of the 
gut form the internal gill-pouch openings ; the other extremity of 
the pouch approaches the external surface of the animal, and finally 
breaks through to form a series of external gill-pouch openings. 

From the mesoblastic tissue, between each gill-pouch, there is 
formed a supporting cartilaginous bar, to which are attached a system 
of branchial muscles, with their nerves and blood-vessels. These 
cartilaginous bars, in all fishes above the Cyclostomata, form a 
supporting framework for the internal gill-slit, so that the gills 
are situated externally to them ; the more primitive arrangement is, 
as already mentioned, a system of cartilaginous bars, extra-branchial 
in position, so that the gills are situated internally to them. 

From this description of the mode of formation of the respiratory 
apparatus in water-breathing vertebrates the conception has arisen 
of the gill-pouch as the branohial unit, a conception which is 
absolutely removed from all idea of a branchial unit such as is 
found in an arthropod, viz. an appendage. 

This conception of spaces as units pervades the whole of embryo- 
logy, and is the outcome of the gastrula theory— a theory whioh 



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1 66 



THE ORIGIN OF VERTEBRATES 



teaches tliat all animals above the Protozoa are derived from a form 
which by invagination of its external surface formed an internal 
cavity or primitive gut. From pouches of this gut other cavities 
were said to be formed, called coelomic cavities, and thus arose the 
group (if coelomatous animals. To speak of the developmental history 
of animals in terms of spaces ; to speak of the atrophy of a cavity 
an though such a thing were possible, is, to my mind, the wrong 
way of looking at the facts of anatomy. It resembles the description 
of a not as a number of holes tied together with string, which is not 
usually considered the best method of description. 

Then? are two ways in which a series of pouches can be formed 
from a simple tube without folding, either by a thinning at regular 
intervals of the original tissue surrounding the tube, or by the 
ingrowth into the tube of the surrounding tissue at regular intervals, 
thus-- 



Mm-. 



DQDaaQ3QDQccQCD3CC323CDQD2| ■aaoaoca&aaaoQcfaiaDaDHsasoQsasa 



QQ0G00QSQCCCCC2aS3aaQ30C:DC3H ■323D3333DDa3aoaD30DCa3DCCD3S 




B 



I 









BccaasBccccccaBcaaQaaccs:: 



:033CCCCC2C23:C3C2:33333C: 



1 2 

Klv., tf?. PlWv.KW*> VO SHOW THK TWO VKIHO^S OV iVlVH-lVRJlATIOX. 

A. by \h* Ihuunuii of tfw nuwbUst At lutorv*U. \X, by tho ingrowth of monoblast at 
inUT\*i*. >*tv % o|Mbl**l ; .V,\v, tuosobUst ; U. .. hv|vblast. 

In tho nrst case ^ tho format ion of a js>vuh is tho significant 
Act. *nd thoroforv tho branchial so S M\\onis nu^ht Iv oxprossod in terms 
4* ivuclw& In tho second c*4o vltt tho formation of a pouch is 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 67 

brought about in consequence of the ingrowth of the mesoblastic 
tissues at intervals ; here, although the end-result is the same as in 
the first case, the pouch-formation is only secondary, the true 
branchial unit is the mesoblastic ingrowth. 

The evidence all points directly to the second method of forma- 
tion. Thus Shipley, in his description of the development of the 
lamprey, says — 

"The gill-slits appear to me to be the result of the ventral 
downgrowth of mesoblast taking place only at certain places, these 
forming the gill-bars. Between each downgrowth the hypoblastic 
lining of the alimentary canal remains in contact with the epiblast ; 
here the gill-opening subsequently appears about the twenty-second 
day." 

Dohrn describes and gives excellent pictures of the growth of 
the diaphragms, as the Ammocoetes grows in size, pictures which 
are distinctly reminiscent of the corresponding illustrations given 
by Brauer of the growth of the internal gills in the scorpion embryo. 

Another piece of evidence confirmatory of the view that the 
branchial segments are really of the nature of internal appendages, 
as the result of which gill-pouches are formed, is given by the presence 
in each of these branchial bars or diaphragms of a separate coelomic 
cavity. From the walls of this cavity the branchial muscles and 
cartilaginous bar are formed. 

Now, from an embryological point of view, the vertebrate shows 
that it is a segmented animal by the formation of somites, which 
consist of a series of divisions of the ccelom, of which the walls form 
a series of muscular and skeletal segments. In the head-region, as 
already mentioned, such coelomic divisions form two rows — a dorsal 
and a ventral set. From the walls of the dorsal set the somatic 
musculature is formed. From those of the ventral set the branchial 
musculature. From the latter also the branchial cartilaginous bars 
are formed. Thus Shipley, in his description of the development 
of the lamprey, says: "The mesoblast between the gills arranges 
itself into head-cavities, and the walls of these cavities ultimately 
form the skeleton of the gill-arches/' 

Similarly, in the arthropod, the segments in the embryo are 
marked out by a series of ccelomic cavities and Kishinouye has 
described in Limulus a separate coelomic cavity for every one of 
the mesosomatic or branchial segments, and he states that in Arachnida 



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* 



1 68 THE ORIGIN OF VERTEBRATES 

the segmental ccelomic cavities extend into the limbs. These 
cavities both in the vertebrate and in the arthropod disappear 
before the adult condition is reached. 

The whole evidence thus points strongly to the conclusion that the 
true branchial segmental units are the branchial bars or diaphragms, 
not the pouches between them. 

It is possible to understand why such prominence has been 
given to the conception of the branchial unit as a gill-pouch rather 
than as a gill-appendage, when the extraordinary change of appear- 
ance in the respiratory chamber of the lamprey which occurs at 
transformation, is taken into consideration. This change is of a 
very far-reaching character, and consists essentially of the formation 
of a new alimentary canal in this region, whereby the pharyngeal 
chamber of Ammoccetes is cut off posteriorly from the alimentary 
canal, and is confined entirely to respiratory purposes, its original 
lumen now forming a tube called the bronchus, which opens into the 
mouth and into a series of branchial pouches. 

In Fig. 68 I give diagrammatic illustrations taken from Nestler's 
paper to show the striking change which takes place at transforma- 
tion, (A) representing three branchial segments of Ammoccetes, and (B) 
the corresponding three segments of Petromyzon. The corresponding 
parts in the two diagrams are shown by the cartilages (br. cart.), the 
sense-organs (S), and the branchial veins ( V. br.) ; the corresponding 
diaphragms are marked by the figures 1, 2, 3 respectively. As is 
clearly seen, it is perfectly possible in the latter case to describe the 
respiratory chamber, as Nestler has done, as divided into a series of 
separate smaller chambers — the gill-pouches — by means of a series 
of diaphragms or branchial bars. The surface of these gill-pouches 
is in part thrown into folds for respiratory purposes, and each gill- 
pouch opens, on the one hand, into the bronchus (Bro.), and, on the 
other, to the exterior by means of the gill-slit. The branchial unit 
in Petromyzon is, therefore, according to Nestler and other mor- 
phologists, the folded opposed surfaces of two contiguous diaphragms, 
and each one of the diaphragms is intersegmental between two gill- 
pouches. 

Nestler then goes on to describe the arrangement in Ammoccetes 
in the same terms, although there is no bronchus or gill-pouch, but 
only an open chamber into which these gill-bearing diaphragms 
project, which open chamber serves both for the passage of food and 




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THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 69 

of the water for respiration. Tliis is manifestly the wrong way to 
look at the matter: the adult form is derived from the larval, not vice 
versa, and the transformation process shows exactly how the gills, 
in Rathke's sense, come together to form the bronchus and so make 
the gill-pouches of Petromyzon. 

When we bear in mind that almost all observers consider that 
the internal branchiae of the scorpion group are directly derived 



V.br. 




VTbr. br.cart. 

ir.cart^ \ 




Fig. 68. — Diagram of thkee Branchial Segments of Ammoccetes (A) compared 
with three Branchial Segments after Transformation (B) to show how 
the Branchial Appendages of Ammocxetes form the Branchial Pouches 
of Petromyzon. (After Nestler.) 

In both figures the branchial cartilages (br. cart.), the branchial view (V. br.) t and the 
sense-organs (&), are marked out in order to show corresponding points. The 
muscles, blood-spaces, branchial arteries, etc., of each branchial segment are 
not distinguished, being represented a uniform black colour. Bro., the bronchus 
into which each gill-pouch opens. 

from branchial appendages of a kind similar to those of Limulus, it 
is evident that a branchial appendage such as that of Ammoccetes 
might also have arisen from such an appendage, because in various 
respects it is easier to compare the branchial appendage of Ammo- 
ccetes, than that of the scorpion group, with that of Limulus. 

In the case of the scorpions, various suggestions have been made 
as to the manner in which such a conversion may have taken place. 
The most probable explanation is that given by Macleod, in which 



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170 THE ORIGIN OF VERTEBRATES 

each of the branchiae of the scorpion group is directly compared 
with the branchial part of the Limulus appendage which has sunk 
into and amalgamated with the ventral surface. 

According to this view, the modification which has taken place in 
transforming the branchial Limulus-appendage into the branchial 
scorpion-appendage is a further stage of the process by which the 
Limulus branchial appendage itself has been formed, viz. the getting 
rid of the free locomotor segments of the original appendage, thus 
confining the appendage more and more to the basal branchial 
portion. So far has this process been carried in the scorpion that 
all the free part of the appendage has disappeared ; apparently, also, 
the intrinsic muscles of the appendage have vanished, with the 
possible exception of the post-stigmatic muscle, so that any direct 
comparison between the branchial appendages of Limulus and the 
scorpions is limited to the comparison of their branchiae, their nerves, 
and their afferent and efferent blood-vessels. 

In the case of Ammoccetes the comparison must be made not 
with air-breathing but with water-breathing scorpions, such as 
existed in past ages in the forms of Eurypterus, Pterygotus, Slimonia, 
and with the crowd of trilobite and Liinulus-like forms which were 
in past ages so predominant in the sea ; forms in some of which the 
branchial appendages had already become internal, but which, from 
the very fact of these forms being water-breathers, probably 
resembled, in respect of their respiratory apparatus, Limulus rather 
than the present-day scorpion. 

On the assumption that the branchial appendages of Ammoccetes, 
like the branchial appendages of the scorpion group, are to a certain 
extent comparable with those of Limulus, it becomes a matter of great 
interest to inquire whether the mode in which respiration is effected 
in Ammoccetes resembles most that of Limulus or of the scorpion. 

The Origin of the Branchial Musculature. 

The difference between the movements of respiration in Limulus 
and those of the scorpions consists in the fact that, although in both 
cases respiration is effected mainly by dorso-ventral muscles, these 
muscles are not homologous in the two cases: in the former, the 
dorso-ventral appendage-muscles are mainly concerned, in the latter, 
the dorso-ventral somatic muscles. 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS • 171 

The paper by Benham gives a full description of the musculature 
of Limulus, and according to his arrangement the muscles are 
divided into two sets, longitudinal and dorso-ventral. Of the 
latter, each mesosomatic segment possesses a pair of dorso-ventral 
muscles, attached to the mid- ventral mesosomatic entochondrite, and 
to the tergal surface (Fig. 58, Dv.). These muscles are called by 
Benham the vertical mesosomatic muscles. I shall call them the 
somatic dorso-ventral muscles, in contradistinction to the dorso- 
ventral muscles of the branchial appendages. Of the latter, the two 
chief are the external branchial (Fig. 66, m^) and the posterior 
entapophysio-branchial (Fig. 66, mi) ; a third muscle is the anterior 
entapophysio-branohial (Fig. 66, ma). Of these muscles, the posterior 
entapophysio-branchial (mi) is closely attached along the branchial 
cartilaginous bar up to its round-headed termination on the anterior 
surface of the appendage. The anterior entapophysio-branchial 
muscle (m^) is attached to the branchial cartilage near the 
entapophysis. 

In the case of the scorpion, as described by Miss Beck, the 
branchial appendage has become reduced to the branchiae, and the 
intrinsic appendage-muscles have entirely disappeared, with the 
possible exception of the small post-stigmatic muscle ; on the other 
hand, the dorso-ventral somatic muscles, which are clearly homolo- 
gous with the corresponding muscles of Limulus, have remained, and 
become the essential respiratory muscles. 

Of these two possible types of respiratory movement it is quite 
conceivable that in the water-breathing scorpions of olden times 
and in their allies, the dorso-ventral muscles of their branchial 
appendages may have continued their rdle of respiratory muscles, and 
so have given origin to the respiratory muscles of the ancestors of 
Ammocoete8. 

The respiratory muscles of Ammocoetes are three in number, and 
have been described by Nestler and Miss Alcock as the adductor 
muscle, the striated constrictor muscle, and the tubular constrictor 
muscle (Fig. 65, m. add., m.c.8., and m.c.L). Of these, the constrictor 
muscle (Fig. 71, m. con. str.) is in close contact with its cartilaginous 
bar, while the adductor (Fig. 71, m. add.) is attached to the cartilage 
only at its origin and insertion, and the tubular muscles (Fig. 71, 
m. con. tub.) have nothing whatever to do with the cartilage at all, 
being attached ventrally to the connective tissue in the neighbourhood 



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I 72 THE ORIGIN OF VERTEBRATES 

of the ventral aorta (V.A.), and dorsally to the mid-line between the 
dorsal aorta (D.A.) and the notochord. 

The close relationship of the constrictor muscle to the carti- 
laginous branchial bar does not favour the surmise that this muscle 
is homologous with the dorso-ventral somatic muscle of the scorpion. 
It is, however, directly in accordance with the view that this muscle 
is homologous with one of the dorso-ventral appendage-muscles, such 
as the posterior entapophysio-branchial muscle (mi, Fig. 66) of the 
Limulus appendage, especially when the homology of the Ammocoetes 
branchial cartilage with the Limulus branchial cartilage is borne in 
mind. I am, therefore, inclined to look upon the constrictor and 
adductor muscles of the Ammocoetes branchial segment as more likely 
to have been derived from dorso-ventral muscles which belonged 
originally to a branchial appendage, such as we see in Limulus, than 
from dorso-ventral somatic muscles, such as the vertical mesosomatic 
muscles which are found both in Limulus and scorpion. In other 
words, I am inclined to hold the view that the somatic dorso-ventral 
muscles have disappeared in this region in Ammocoetes, while dorso- 
ventral appendage-muscles have been retained, i.e. the exact reverse 
to what has taken place in the air-breathing scorpion. 

I am especially inclined to this view because of the manner in 
which it fits in with and explains van Wijhe's results. Ever since 
Schneider divided the striated muscles of vertebrates into parietal 
and visceral, such a division has received general acceptance and, as 
far as the head-region is concerned, has received an explanation in 
van Wijhe's work ; for Schneiders grouping corresponds exactly to the 
two segmentations of the head-mesoblast, discovered by van Wijhe, 
i.e. to the somatic and splanchnic striated muscles according to my 
nomenclature. Of these two groups the splanchnic or visceral 
striated musculature, innervated by the Vth, Vllth, IXth, and Xth 
nerves, which ought on this theory to be derived from the muscu- 
lature of the corresponding appendages, is, speaking generally, dorso- 
ventral in direction in Ammoccetes and of the same character through- 
out ; the somatic musculature, on the other hand, is clearly divisible, 
in the head region, into two sets — a spinal and a cranial set. The 
somatic muscles innervated by the spinal set of nerves, including in 
this term the spino-occipital or so-called hypoglossal serves, are in 
Amnioctutes most sharply defined from all the other muscles of 
the body. They form the great dorsal and ventral longitudinal 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 73 

body-muscles, which extend dorsally as far forward as the nose and 
are developed embryologically quite distinctly from the others, being 
formed as muscle-plates (Kastchen). On the other hand, the cranial 
somatic muscles are the eye-muscles, the formation of which resembles 
that of the visceral muscles, and not of the spinal somatic. Their 
direction is not longitudinal, but dorso-ventral ; they cannot, in my 
opinion, be referred to the somatic trunk- muscles, and must, therefore, 
form a separate group to themselves. Thus the striated musculature 
of the Ammocoetes must be divided into (1) the visceral muscles ; 
(2) the longitudinal somatic muscles ; and (3) the dorso-ventral somatic 
muscles. Of these the 1st, on the view just stated, represent the 
original appendage-muscles ; the 2nd belong to the spinal region, and 
will be considered with that region ; the 3rd represent the original 
segmental dorso-ventral somatic muscles, which are so conspicuous 
in the musculature of the Limulus and the scorpion group. 

The discussion of this last statement will be given when I come 
to deal with the prosomatic segments of Ammoccetes. I wish, here, 
simply to point out that van Wijhe has shown that the eye-muscles 
develop from his 1st, 2nd, and 3rd dorsal mesoblastic segments, and 
therefore represent the somatic muscles belonging to those segments, 
while no development of any corresponding muscles takes place in 
the 4th, 5th, and 6th segments ; so that if the eye-muscles represent 
a group of dorso-ventral somatic muscles, such muscles have been 
lost in the 4th, 5th, and 6 th segments. The latter segments are, 
however, the glossopharyngeal and vagus segments, the branchial 
musculature of which is derived from the ventral segments of the 
mesoderm. In other words, van Wijhe's observations mean that the 
dorso-ventral somatic musculature has been lost in the branchial or 
mesosomatic region, while the dorso-ventral appendage musculature 
has been retained, and that, therefore, the mode of respiration in 
Ammocoetes more closely resembles that of Limulus than of Scorpio. 

In addition to these branchial muscles, another and very striking 
set of muscles is found in the respiratory region of Ammocoetes— the 
so-called tubular muscles. These muscles are of great interest, but 
as they are especially connected with the Vllth nerve, their con- 
sideration is best postponed to the chapter dealing with that nerve. 

Also, in connection with the vagus group of nerves, special sense- 
organs are found in the skin covering this mesosomatic region, the 
so-called epithelial pit-organs (Ep. pit., Fig. 71). They, too, are of 



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174 THE ORIGIN OF VERTEBRATES 

great interest, but their consideration may also better be deferred to 
the chapter dealing with those special sense-systems known as the 
lateral line and auditory systems. 



Comparison of the Branchial Circulation in Ammoccetes and 

Limulus. 

Closely bound up with the respiratory system is the nature of 
the circulation of blood through the gills. Before, therefore, proceeding 
to the consideration of the segments in front of those which carry 
branchiae, it is worth while to compare the circulation of the blood 
in the gills of Limulus and of Ammoccetes respectively. 

In all the higher vertebrates the blood circulates in a closed 
system of capillaries, which unite the arterial with the venous systems. 
In all the higher invertebrates this capillary system can hardly be 
said to exist ; the blood is pumped from the arterial system into blood, 
spaces or lacunae, and thus comes into immediate contact with the 
tissues. From these it is collected into veins, and so returned to the 
heart. There is, in fact, no separate lymph-system in the higher 
invertebrates ; the blood-system and lymph-system are not yet 
differentiated from each other. This also is the case in Ammocoetes ; 
here, too, in many places the blood is poured into a lacunar space, 
and collected thence by the venous system ; a capillary system is 
only in its commencement and a lymph-system does not yet exist. 
In this part of its vascular system Ammocoetes again resembles the 
higher invertebrates more than the higher vertebrates. 

This resemblance is still more striking when the circulation 
in the respiratory organs of the two animals is compared. A 
branchial appendage is essentially an appendage whose vascular 
system is arranged for the special purpose of aerating blood. In the 
higher vertebrates such a purpose is attained by the pulmonary 
capillaries, in Limulus by the division of the posterior surface of the 
basal part of the appendage into thin lamellar plates, the interior of 
each of which is filled with blood. The two surfaces of each lamella 
are kept parallel to each other by means of fibrous or cellular strands 
forming little pillars at intervals, called by Macleod " colonetteR." 
A precisely similar arrangement is found in the scorpion gill-lamella, 
as seen in Fig. 69, A, taken from Macleod. In Ammocoetes there are 
no well-defined branchial capillaries, but the blood circulates, as in 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS 



175 



the invertebrate gill, in a lamellar space ; here, also, as Nestler has 
shown, the opposing walls of the gill-lamella are held in position by 
little pillar-like cells, as seen in Fig. 69, B, taken from his paper. 

In this representative of the earliest vertebrates the method of 
manufacturing an efficient gill out of a lacunar blood-space is pre- 
cisely the same as that which existed in Limulus and the scorpion, 
and, therefore, as that which existed in the dominant invertebrate 
group at the time when vertebrates first appeared. This similarity 
indicates a close resemblance between the circulatory systems of the 
two groups of animals, and therefore, to the superficial inquirer, would 
indicate an homology between the heart of the vertebrate and the 
heart of the higher inverte- 
brate; but the former is situ- 
ated ventrally to the gut and 
the nervous system, while the 
latter is composed of a long 
vessel which lies in the mid- 
dorsal line immediately under 
the external dorsal covering. 
Indeed, this ventral position of 
the heart in the one group of 
animals and its dorsal position 
in the other, combined with 
the corresponding positions of 
the central nervous system, is 
one of the principal reasons 
why all the advocates of the 
origin of vertebrates from the 
Appendiculata, with the single exception of myself, feel compelled to 
reverse the dorsal and ventral surfaces in deriving the vertebrate 
from the invertebrate. But there is one most important fact which 
ought to make us hesitate before accepting the homology of the 
dorsal heart of the arthropod with the ventral heart of the vertebrate 
— The heart in all invertebrates is a systemic heart, i.e, drives the 
arterial blood to the different organs of the body, and then the veins 
cany it back to the respiratory organ, from whence it passes to the 
heart. 

The only exception to this scheme is found in the vertebrate 
where the heart is essentially a branchial heart, the blood being 





A ■ 



Fig. 69.— Comparison op Branchial La- 
mellae of Limulus and Scorpio with 
Branchial Lamella of Ammocostes. 

A, Branchial lamellae of Scorpio (after 
Macleod) ; B, Branchial lameUte of Am- 
moccetes (after Nestler). 



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176 THE ORIGIN OF VERTEBRATES 

driven from the heart to the ventral aorta, from which by the 
branchial arteries it is carried to the gills, and then, after aeration, is 
collected into the dorsal aorta, whence it is distributed over the 
body. The distributing systemic vessel is the dorsal aorta, not the 
heart which belongs essentially to the ventral venous system. This 
constitutes a very strong reason for believing that the systemic heart 
of the invertebrate is not homologous with the heart of the vertebrate. 
How, then, did the vertebrate heart arise ? 

Let us first see how the blood is supplied to the gills in Limulus. 

In Limulus the blood flows into the lamellae from sinuses or 
blood-spaces (b.s. t Fig. 66) at the base of each of the lamellae, which 
sinuses are filled by a vessel which may be called the branchial 




Fig. 70.— Longitudinal Diagbammatic Section through the Mesobomatic 
Region op Limulus, to show the origin op the Branchial Arteries. 
(After Benham.) 

L.V.S., longitudinal venous sinus, or collecting sinus; a. br. t branchial arteries; 
V.p., veno-pericardial muscles ; P., pericardium. 

artery, since it is the afferent branchial vessel. On each side of the 
middle line of the ventral surface of the body a large longitudinal 
venous sinus exists, called by Milne-Edwards the venous collecting 
sinus, L.V.S., (Fig. 70 and Fig. 58), which gives off to each of the 
branchial appendages on that side a well-defined afferent branchial 
vessel — the branchial artery (a. h\). The blood of the branchial artery 
flows into the blood-spaces between the anterior and posterior 
laminae of the appendage and thence into the gill-lamellae, from 
which it is collected into an efferent vessel or branchial vein, termed 
by Milne-Edwards the branchio-cardiac canal, which carries it back 
to the dorsal heart. The position of the branchial artery and vein 
is shown in Fig. 66, which represents a section through the branchial 
appendage of Limulus at right angles to the cartilaginous branchial 
bar (br. cart), just as Fig. 65 represents a section through the 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS I 77 

branchial appendage of Ammoccetes at right angles to the carti- 
laginous branchial bar. 

Further, the observations of Blanchard, Milne -Ed wards, Eay 
Lankester, and Benham concur in showing that in both Limulus and 
the scorpion group a striking and most useful connection exists 
between the heart and these two collecting venous sinuses, in the 
shape of a segmentally arranged series of muscular bands ( V.p., Fig. 
70 and Fig. 58), attached, on the one hand, to the pericardium, and 
on the other to the venous collecting sinus on each side. These 
muscular bands, to which Lankester and Benham have given the 
name of 'veno-pericardial muscles/ are so different in appearance 
from the rest of the muscular substance, that Milne-Edwards did not 
recognize them as muscular, but called them ' brides transparentes.' 
Blanchard speaks of them in the scorpion as 'ligaments con- 
tractiles/ and considers that they play an important part in assisting 
the pulmonary circulation ; for, he says, " en mettant a nu une portion 
du coeur, on remarque que ces battements se font sentir sur les liga- 
ments contractiles, et determinent sur les poches pulmonaires une 
pression qui fait aussitot refluer et remonter le sang dans les vaisseaux 
pneumocardiaques." Lankester, in discussing the veno-pericardial 
muscles of Limulus and of the scorpions, says that these muscles 
probably contract simultaneously with the heart and are of great 
importance in assisting the flow through the pulmonary system. 
More recently Carlson has investigated the action of these muscles 
in the living Limulus and found that they act simultaneously with 
the muscles of respiration. 

Precisely the same arrangement of veno-pericardial muscles and 
of longitudinal venous collecting sinuses occurs in the scorpions. It 
is one of the fundamental characters of the group, and we may fairly 
assume that a similar arrangement existed in the extinct forms from 
which I imagine the vertebrate to have arisen. The further con- 
sideration of this group of muscles mil be given in Chapter IX. 

Passing now to the condition of the branchial blood-vessels of 
Ammocoetes, we see that the blood passes into the gill-lamelloe from a 
blood-space in the appendage, which can hardly be dignified by the 
name of a blood-vessel. This blood-space is supplied by the branchial 
artery which arises segmentally from the ventral aorta (V.A.), as seen 
in Fig. 71 (taken from Miss Alcock's paper). From the gill-lamellae 
the blood is collected into an efferent or branchial vein (v. br.), which 

x 



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1 7 8 



THE ORIGIN OF VERTEBRATES 



runs, as seen in Fig. 65, along the free edge of the diaphragm, and 
terminates in the dorsal aorta. 

The ventral aorta is a single vessel near the heart, but at the com- 
mencement of the thyroid it divides into two, and so forms two ventral 
longitudinal vessels, from which the branchial arteries arise segmen tally. 










Fig. 71.— Diagram constructed from a series op Transverse Sections through 
a Branchial, Segment, showing the arrangement and relative positions 
op the Cartilage, Muscles, Nerves, and Blood- Vessels. 

Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured 
blue are the internal sensory nerves to the diaphragms and the external sensory 
nerves to the sense-organs of the lateral line system. Br. cart., branchial 
cartilage; M. con. str. t striated constrictor muscles; M. con. tub., tubular 
constrictor muscles ; M. add., adductor muscle ; D.A., dorsal aorta ; V.A., ventral 
aorta; S., sense-organs on diaphragm; n. hat., lateral line nerve; X, epibran- 
chial ganglia of vagus ; B. br. prof. VII., ramus branchialis profundus of facial ; 
J. v., jugular vein ; Ep. pit., epithelial pit. 

From this description it is clear that the vascular supply of the 
branchial segment of Ammoccctes would resemble most closely the 
vascular supply of the Limulus branchial appendage, if the ventral 
aorta of the former was derived from two longitudinal veins, homo- 
logous with the paired longitudinal venous sinuses of the latter. 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS I 79 

A priori, such a derivation seems highly improbable ; and yet it 
is precisely the manner in which embryology teaches us that the 
heart and ventral aorta of the vertebrate have arisen. 



The Origin of the Invertebrate Heart and the Origin of the 
Vertebrate Heart. 

Not only does the vertebrate heart differ from that of the inverte- 
brate, in that it is branchial while the latter is systemic, but also it 
is unique in its mode of formation in the embryo. In the Appen- 
diculata the heart is formed as a single organ in the mid-dorsal line 
by the growth of the two lateral plates of mesoblast dorsalwards, 
the heart being formed where they meet. In Mammalia and Aves, 
the heart and ventral aorta commence as a pair of longitudinal veins, 
one on each side of the commencing notochord. 

If the embryo be removed from the yolk, the surface of the embryo 
covering these two venous trunks can be spoken of as the ventral 
surface of the embryo at that stage, and indeed we find that in the 
present day there is an increasing tendency to speak of this surface 
as the ventral surface of the embryo. Thus, Mitsukuri, in his studies 
of chelonian embryos, lays great stress on the importance of surface 
views and when the embryo has been removed from the yolk, 
figures and speaks of its ventral surface. So, also, Locy and Neal 
find that the best method of seeing the early segments of the embryo 
is to remove the embryo from the yolk, and examine what they speak 
of as a ventral view. At the period, then, before the formation of the 
throat, we may say that on the ventral surface of the embryo a pair 
of longitudinal venous sinuses are found, one on each side of the mid- 
ventral line, which are in the same position with respect to the mid- 
axis of the embryo as are the longitudinal venous sinuses in Limulus. 

The next step is the formation of the throat by the extension of 
the layers of the embryo laterally to meet in the mid-line and so 
form the pharynx, with the consequence that a new ventral surface is 
formed ; these two veins, as is well known, travel round also, and, 
meeting together in the new mid-ventral line, form the subintestinal 
vein, the heart, and the ventral aorta. 

What is true of Mammalia and Aves, has been shown by P. Mayer 
to be true universally among vertebrates, so that in all cases the heart 
and ventral aorta have arisen by the coalescence in the new mid- ventral 



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i8o 



THE ORIGIN OF VERTEBRATES 




line of two longitudinal venous channels, which were originally situ- 
ated one on each side of the notochord, in what was then the ventral 

surface of this part of the embryo. 
This history is especially in- 
structive in showing how the 
pharyngeal region is formed by 
the growing round of the lateral 
mesoblast, i.e. the muscular and 
other mesoblastic tissues of the 
branchial segments, and how the 
two longitudinal veins take part 
in this process. The phyloge- 
netic interpretation of this em- 
bryological fact seems to be, 
that the new ventral surface of 
the vertebrate in this region is 
formed, not only by the branchial 
appendages, but also by the 
growth ventrally of that part 
of the original ventral surface 
which covered each longitudinal 




CXLS 



LV.S* 



Fig. 72.— Diagram (Upper Half of 
Figure) of the Original Position 
of Veins (H) which come together 

TO FORM THE HEART OF A VERTEBRATE. 



C.N. S.j central nervous system; nc. t 
notochord ; m., myotome. 

The lower half of figure shows compara- 
tive position of the longitudinal venous 
sinus (L.V.S.) in Limulus. C.N.S., 
central nervous system ; ,4/., alimentary 
canal ; H. y heart ; m., body-muscles. 



venous sinus. 

The following out of the 
consecutive clues, which one 
after the other arise in har- 
monious succession as the neces- 
sary sequence of the original 
working hypothesis, brings even now into view the manner in which 
the respiratory portion of the alimentary canal arose, and gives 
strong hints as to the position of that part of the arthropod which 
gave origin to the notochord. Here I will say no more at present, 
for the origin of the new alimentary canal of the vertebrate and of 
the notochord will be more fittingly discussed as a whole, after all 
the other organs of the vertebrate have been compared with the 
corresponding organs of the arthropod. 

The strong evidence that the vertebrate heart was formed from a 
pair of longitudinal venous sinuses on the ventral side of the central 
canal, carries with it the conclusion that the original single median 
dorsal heart of the arthropod is not represented in the vertebrate, 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS l8l 

for the dorsal aorta cannot by any possibility represent that 
heart. 

Although it is not now functional the original existence of so 
important an organ as a dorsal heart may have left traces of its 
former presence ; if so, such traces would be most likely to be visible 
in the lowest vertebrates, just as the median eyes are much more 
evident in them than in the higher forms. In Fig. 58 the position of 
the dorsal heart is shown in Limulus, and in Fig. 70 the shape and 
extent of this dorsal heart is shown. It extends slightly into the pro- 
somatic region, and thins down to a point there, runs along the length 
of the animal and finally thins down to a point at the caudal end. 

The heart is surrounded by a pericardium, from which at regular 
intervals a number of dorso-ventral muscles pass, to be inserted into 
the longitudinal venous sinus on each side. These veno-pericardial 
muscles are absolutely segmental with the mesosomatic segments, 
and are confined to that region, with the exception of two pairs in the 
prosomatic region. Their homologies will be discussed later. 

Any trace of a heart such as we have just described must be 
sought for in Ammoccetes between the central nervous system and 
the mid-line dorsally. Now, in this very position a large striking 
mass of tissue is found, represented in section in Fig. 73, /. It 
forms a column of similar tissue along the whole mid-dorsal region, 
except at the two extremities; it tapers away in the caudal region, 
and headwards grows thinner and thinner, so that no trace of it is 
seen anterior to the commencement of the branchial region. It 
resembles in its dorsal position, in its shape, and in its size a dorsal 
heart-tube such as is seen in Limulus and elsewhere, but it differs 
from such a tube in its extension headwards. The heart-tube of 
Limulus ceases at the anterior end of the mesosomatic region, this 
fat-column of Ammoccetes at the posterior end. In its structure there 
is not the slightest sign of anything of the nature of a heart ; it is 
a solid mass of closely compacted cells, and the cells are all very 
full of fat, staining intensely black with osmic acid. Nowhere else 
in the whole body of Ammoccetes is such a column of fat to be found. 
It is not skeletogenous tissue with cells of the nature of cartilage- 
cells, as Gegenbaur thought and as Balfour has depicted (VoL II., 
Fig. 315) in his ' Comparative Embryology/ as though this tissue were 
a part of the vertebral column, but is simply fat-cells, such as might 
easily have taken the place of some other previously existing organ. 



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l82 



THE ORIGIN OF VERTEBRATES 



I do not know liow to decide the question which thus arises. 
Supposing, for the sake of argument, that this column of fat-cells 
has really taken the place of the original dorsal heart, what criterion 
would there be as to this ? The heart ex hypothesi having ceased to 

function, the muscular tissue 
would not remain, and the 
space would be filled up, 
presumably with some form 
of connective tissue. As 
likely as not, the connective 
tissue might take the form 
of fatty tissue, the storage 
of fat being a physiological 
necessity to an animal, while 
at the same time no special 
organ has been developed 
for such a purpose, but fat 
is being laid down in all 
manner of places in the 
body. 

This dorsal fat-column, 
as it is seen in Ammoccetes, 
is not found in the higher 
vertebrates, so that it pos- 
sesses, at all events, the 
significance of being a pecu- 
liarity of ancient times 
before the vertebrate skele- 
tal column was formed. 

I mention it here in 
connection with my view 
as to the origin of verte- 
brates, because there it is, 
in the very place where the 
dorsal heart ought to have been. For my own part, I should not have 
expected that a muscular organ such as the heart would leave any 
trace of itself if it disappeared, so that its absence in the dorsal region 
of the vertebrate does not seem to me in the slightest degree to 
invalidate my theory. 




Fig. 78. — Section through the Notochobd 
{nc.) t the Spinal Canal and the Fat- 
column (/.), of Ammocostes, drawn from 
an Osmic Preparation. 

»p. c, spinal oord; gl. t glandular tissue filling 
the spinal canal; «&., Gegenbaur's skeleto- 
genous cells ; p., pigment. 



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THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 83 

Summary. 

From the close similarity of structure and position between the branchial 
skeleton of Limulus and of Ammocoetes, as given in the preceding chapter, it 
logically follows that the branchiae of Ammocoetes must be homologous with the 
branchiae of Limulus. But the respiratory apparatus of Limulus consists of 
branchial appendages. It follows, therefore, that the branchiae of Ammocoetes, 
and consequently of the vertebrates, must have been derived from branchial 
appendages, and as they are internal, not external, such branchial appendages 
must have been of the nature of ' sunk-in * branchial appendages. Such 
internal appendages are characteristic of the scorpion tribe, and of, perhaps, 
the majority of the Palaeostraca, for no external respiratory appendages have 
been discovered in any of the sea-scorpions. 

In the vertebrates— and it is especially well shown in Ammocoetes — a double 
segmentation exists in the head-region, a body or somatic segmentation, and 
a branchial or splanchnic segmentation, respectively expressed by the terms 
mesomeric and branchiomeric segmentations. The nerves which supply the 
latter segments form a very well-marked group (Charles Bell's system of lateral 
or respiratory nerves) which do not conform to the system of spinal nerves, for 
they do not arise from separate motor and sensory roots, but are mixed nerves 
from the very beginning. 

The system of cranial segmental nerves is older than the spinal system, and 
cannot, therefore, be derived from it, but can be arranged as a system supplying 
two segments, somatic and splanchnic, which differ in the following way : Each 
somatic segment is supplied by two roots, motor and sensory respectively, as in 
the spinal cord segments, while each splanchnic segment possesses only one root, 
which is mixed in function. 

The peculiarities of the grouping of the cranial segmental nerves, which 
have hitherto been unexplained, immediately receive a straightforward and 
satisfactory explanation if the splanchnic or branchiomeric segments owe their 
origin to a system of appendages after the style of those of Limulus. 

In Limulus and all the Arthropoda, the segmentation is double, being com- 
posed of (1) somatic or body-segments, constituting the mesomeric segmentation ; 
(2) appendage-segments, which, seeing that they carry the branchiae, constitute 
a branchiomeric segmentation. Similarly to the cranial region of the vertebrate, 
the nerves which supply the somatic segments arise from separate sensory and 
motor roots, while the single nerve which supplies each appendage contains all 
the fibres for the appendage, both motor and sensory. 

It follows from this that the branchial segments supplied by the vagus 
and glossopharyngeal nerves ought to have arisen from appendages bearing 
branchiae. 

Although the evidence of such appendages has entirely disappeared in the 
higher vertebrates, together with the disappearance of branchiae, and is not 
strikingly apparent in the higher gill-bearing fishes, yet in Ammocoetes, so 
great is the difference here from all other fishes, it is natural to describe the 
pharyngeal or respiratory chamber as a chamber into which a symmetrical series 
of respiratory appendages, the so-called diaphragms, are dependent. Each of 
these appendages possesses its own mixed nerve, glossopharyngeal or vagus, 



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184 THE ORIGIN OF VERTEBRATES 

its own cartilage, its own set of visceral muscles, its own sense-organs, just as 
do the respiratory appendages of Limulus. 

The branchial unit in the vertebrate is not the gill-pouch, but the branchial 
bar or appendage between the pouches. Embryology shows how each such 
appendage grows inwards, how a coelomic cavity is formed in it, similarly to the 
ingrowing of the branchial appendage in scorpions. 

We do not know how the palteostracan sea-scorpions breathed ; they resemble 
the scorpion of the present day somewhat in form, but they are in many respects 
closely allied to Limulus. The present-day scorpion is a land animal, and the 
muscles by which he breathes are dorso-ventral somatic muscles, while those of 
Limulus are the appendage-muscles. 

The old sea-scorpions very probably used their appendage-muscles after the 
Limulus fashion, being water-breathers, even although their respiratory appen- 
dages were no longer free but sunk in below the surface of the body. The 
probability that such was the case is increased after consideration of the method 
of breathing in Ammocoetes, for the respiratory muscles of the latter animal are 
directly comparable with the muscles of the respiratory appendages of Limulus, 
and are not somatic. Even the gills themselves of Ammocoetes are built up in 
the same fashion as are those of Limulus and the scorpions. The conception of 
the branchial unit as a gill-bearing appendage, not a gill-pouch, immediately 
explains the formation of the vertebrate heart, which is so strikingly different 
from that of all invertebrate hearts, in that it originates as a branchial and 
not as a systemic heart, and is formed by the coalescence of two longitudinal 
veins. 

The origin of these two longitudinal veins is immediately apparent if the 
vertebrate arose from a palseostracan, for in Limulus and the whole scorpion 
tribe, in which the heart is a systemic heart, the branchiae are supplied with 
blood from two large longitudinal venous sinuses, situated on each side of the 
middle line of the animal in an exactly corresponding position to that of the two 
longitudinal veins, which come together to form the heart and ventral aorta of 
the vertebrate. The consideration of the respiratory apparatus and of its blood- 
supply in the vertebrate still further points to the origin of vertebrates from the 
Palaeostraca. 



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CHAPTER V 

THE EVIDENCE OF THE THYROID GLAND 

The value of the appendage-unit in non-branchial segments. — The double nature 
of the hyoid segment. — Its branchial part. — Its thyroid part. — The double 
nature of the opercular appendage. — Ita branchial part. — Its genital part. 
— Unique character of the thyroid gland of Ammocoetes — Its structure. — 
Its openings. — The nature of the thyroid segment. — The uterus of the 
scorpion. — Its glands. — Comparison with the thyroid gland of Ammocoetes. 
— Cephalic genital glands of Limulus. — Interpretation of glandular tissue 
filling up the brain-case of Ammocoetes. — Function of thyroid gland. — 
Relation of thyroid gland to sexual functions. — Summary. 

I have now given my reasons why I consider that the glosso- 
pharyngeal and vagus nerves were originally the nerves belonging to 
a series of mesosoinatic branchial appendages, each of which is still 
traceable in the respiratory chamber of Ammocretes, and gives the 
type-form from which to search for other serially homologous, 
although it may be specially modified, segments. 

As long as the branchial unit consisted of the gill-pouch the 
segments of the head-region were always referred to such units, 
hence we find Dohrn and Marshall picturing to themselves the 
ancestor of vertebrates as possessing a series of branchial pouches 
right up to the anterior end of the body. Marshall speaks of 
olfactory organs as branchial sense-organs ; Dohrn of the mouth as 
formed by the coalescence of gill-slits, of the trigeminal nerve 
as supplying modified branchial segments, etc. ; thus a picture of 
an animal is formed such as never lived on this earth, or could be 
reasonably imagined to have lived on it. Yet Dohrn's conceptions 
of the segmentation were sound, his interpretation only was in 
fault, because he was obliged to express his segments in terms of 
the gill-pouch unit. Once abandon that point of view and take as 
the unit a branchial appendage, then immediately we see that in 
the region in front of the branchise we may still have segments 



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1 86 THE ORIGIN OF VERTEBRATES 

homologous to the branchial segments, originally characterized by 
the presence of appendages, but that such appendages need never 
have carried branchiae. The new mouth may have been formed by 
such appendages, which would express Dohrn's suggestion of its 
formation by coalesced gill-slits ; the olfactory organ may have been 
the sense-organ belonging to an antennal appendage, which would 
be what Marshall really meant in calling it a branchial sense-organ. 

The Facial Nerve and the Foremost Eespiratory Segment. 

This simple alteration of the branchiomeric unit from a gill-pouch 
to an appendage, which may or may not bear branchiae, immedi- 
ately sheds a flood of light on the segmentation of the head-region, 
and brings to harmony the chaos previously existing. Let us, then, 
follow out its further teachings. Next anteriorly to the glosso- 
pharyngeal and vagus nerves comes the facial nerve ; a nerve which 
supplies the hyoid segment, or, rather, according to van Wijhe the two 
hyoid segments, for embryologically there is evidence of two segments. 
As already mentioned, the facial nerve is usually included in the 
trigeminal or pro-otic group of nerves, the opisthotic group being 
confined to the glossopharyngeal and vagus. This inclusion of the 
facial nerve into the pro-otic group of nerves forms one of the main 
reasons why this group has been supposed to have originally supplied 
gill-pouch segments, for the hyoid segment is clearly associated with 
branchiae. 

When, however, we examine Ammocoetes (c/. Figs. 63 and 64) 
it is clear that the foremost of the segments forming the respiratory 
chamber, which must be classed with the rest of the mesosomatic or 
opisthotic segments, is that supplied by the facial nerves. 

An examination of this respiratory chamber shows clearly that 
there are six pairs of branchial appendages or diaphragms, which are 
all exactly similar to each other. These are those already considered, 
the foremost of which are supplied by the IXth or glossopharyngeal 
nerves. Immediately anterior to this glossopharyngeal segment is 
seen in the figures the segment supplied by the Vllth or facial 
nerves. It is so much like the segments belonging to the glosso- 
pharyngeal and vagus nerves as to make it certain that we are dealing 
here with a branchial segment, composed of a pair of branchial 
appendages similar to those in the other cases, except that the 



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Respiratory jAp pen 
9 Nerve SuppI 




---> Pigment 



Fig. 74. — Vextkal h 
of Hkaij-regiox of 
moccetes. 



Somatic muscles coloured 
red. Branchial and visce- 
ral muscles coloured blue. 
Tubular constrictor mus- 
cles distinguished from 
Ftriated constrictor mus- 
clea by simple batching. 
Tin!,, tentacles ; Tent, wi.c, 
muco-cartilage of tenta- 
cles; Vcl. m.c, muco-car- 
tilage of the velum ; Hy. 
tii.c., muco-cartilagc of the 
hyoid segment; Ps. 6r., 
pseudo-branchial groove ; 
Br, cart,i branchial carti- 
lages; Sp., space between 
Homatic and splanchnic 
muscles ; Th. op., orifice of 
thyroid ; //., heart. 



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1 88 THE ORIGIN OF VERTEBRATES 

cartilaginous bar is here replaced by a bar of muco-cartilage and 
the branchiae are confined to the posterior part of each appendage. 
The anterior portion is, as is seen in Fig. 74, largely occupied by 
blood-spaces, but in addition carries the ciliated groove (ps. br.) called 
by Dohrn ' pseudo-branchiale Rhine/ This groove leads directly 
into the thyroid gland, which is a large bilateral organ situated in 
the middle line, as seen in Fig. 80 and Fig. 85. As shown by Miss 
Alcock, the facial nerve supplies this thyroid gland, as well as the 
posterior hyoid branchial segment, and, as pointed out by Dohrn, 
there is every reason to consider this thyroid gland as indicative of 
a separate segment, especially when van Wijhe's statement that the 
hyoid segment is in reality double is taken into account. 

The evidence, then, of Ammocoetes points directly to this con- 
clusion: The facial nerves represent the foremost of the mesoso- 
matic group of nerves, and supply two segments, which have amalga- 
mated with each other. The most posterior of these, the hyoid 
segment, is a branchial segment of the same character as those 
supplied by the vagus and glossopharyngeal nerves ; represents, 
therefore, the foremost pair of branchial appendages. The anterior or 
thyroid segment, on the other hand, differs from the rest in that, 
instead of branchiae, it carries the thyroid gland with its two ciliated 
grooves. If this segment, which is the foremost of the mesosomatic 
segments, also indicates a pair of appendages which carry the thyroid 
gland instead of branchiae, then it follows that this pair of appendages 
has joined together in the mid-line ventrally and thus formed a 
single median organ — the thyroid gland. If, then, we find that the 
foremost of the mesosomatic appendages in the Palaeostraca was really 
composed of two pairs of appendages, of which the most posterior 
carried branchiae, while the anterior pair had amalgamated in the 
mid-line ventrally, and carried some special organ instead of 
branchiae, then the accumulation of coincidences is becoming so 
strong as to amount to proof of the correctness of our line of 
investigation. 



The First Mesosomatic Segment ik Limulus and its Allies. 

What, then, is the nature of the foremost pair of mesosomatic 
appendages in Limulus. They differ from the rest of the mesosomatic 
appendages in that they do not carry branchiae, and instead of being 



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THE EVIDENCE OF THE THYROID GLAND 



189 



separate are joined together in the mid-line ventrally to form a single 
terminal plate-like appendage known as the operculum. On its 
posterior surface the operculum carries the genital duct on each side. 

So also in the scorpion group, the operculum is always found 
and always carries the genital ducts. 

A survey of the nature of the opercular appendage demonstrates 
the existence of three different types — 

1. That of Limulus, in which the operculum is free, and carries 
only the terminations of the genital ducts. In this type the duct on 
each side opens to the exterior separately (Fig. 75). 

2. The type of Scorpio, Androctonus, Buthus, etc., in which the 





Fig. 75.— Operculum op Limulus to 
show the two separate genital 
Ducts. 



Oea. doot 



Fig. 76. 



Geo. daet. 



■ Operculum 
Scorpion. 



of Male 



Ut. t terminal chamber, or uterus. 



operculum is not free, but forms part of the ventral surface of the 
body-wall, but, like Limulus, carries only the terminations of 
the genital ducts. In this type the duct on each side terminates 
in a common chamber (vagina or uterus), which communicates with 
the exterior by a single external median opening. This common 
chamber, or uterus ( Ut.), extends the whole breadth of the operculum 
(as seen in Fig. 76), and is limited to that segment. 

3. The type of Thelyphonus, Hypoctonus, Phrynus, and other 
members of the Pedipalpi, in which the operculum forms a part 
of the ventral surface of the body wall, but no longer covers only 
the termination of the genital apparatus. It really consists of two 
parts, a median anterior, which covers the terminal genital apparatus, 



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190 



THE ORIGIN OF VERTEBRATES 



Ut. Maso. 



Int Op. 



Ext Op. 



and a lateral posterior, which covers the first pair of gills, or lung- 
books, as they are called. In this type (Fig. 77) the genital ducts 
terminate in a common chamber or uterus, the nature of which will 
be further considered. 

As has been pointed out by Blanchard, the terminal genital 
organs of the scorpions and the Pedipalpi vary considerably in the 
different genera, especially the male genital organs. The general 
type of structure is the same, and consists in both male and female 
of vasa deferentia, which come together to form a common chamber 

before the actual opening 
to the exterior. This com- 
mon chamber has been 
called in the female scor- 
pion the vagina, or in 
Thelyphonus the uterus. 
I shall use the latter term, 
in accordance with Tar- 
nani's work, and the corre- 
sponding chamber in the 
male will be the uterus 
mascvlinns. 

A considerable discus- 
sion has taken place about 
the method of action of the 
external genital organs in 
the members of the scorpion 
tribe, into which it is hardly 
necessary to enter here. 
The evidence points to the 
conclusion that in all these forms the operculum covers a median 
single chamber or uterus, into which the genital ducts open on each 
side, the main channels of emission being provided with a massive 
chitinous internal framework. We may feel certain that in the old 
extinct sea-scorpions, Eurypterus, etc., a similar arrangement existed, 
and that therefore in them also the median portion of the operculum 
covered a median chamber or uterus composed of the amalgamation 
of the terminations of the two genital ducts, which were originally 
separate, as in Limulus. 

The observations of Schmidt, Zittel, and others show that the 




Fig. 77.— Operculum and Following Seg- 
ments op Male Thelyphonus. 

Opercular segment is marked out by thick black 
line. Ut. Masc. y uterus masculinus ; Int. Op. t 
internal opening of uterus into genital chamber ; 
Ext. Op., common external opening to genital 
chamber (Qen. Ch.) and pulmonary chamber. 



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THE EVIDENCE OF THE THYROID GLAND 191 

operculum in the old extinct sea-scorpions, Eurypterus, Pterygotus, 
etc., belonged to the type of Thelyphonus, rather than to that of 
Limulus or Scorpio. In Fig. 78 I give a picture from Schmidt of the 
ventral aspect of Eurypterus, and by the side of it a picture of the 
isolated operculum. Schmidt considers that there were five branchiae- 
bearing segments constituting the mesosoma, the foremost of which 
formed the operculum. Such operculum is often found isolated, and 
is clearly composed of two lateral 
appendages fused together in the 
middle line, of such a nature as to 
form a median elongated tongue, 
which lies between and separates 
the first three pairs of branchial 
segments. This median tongue, 
together with the anterior and 
median portion of the operculum, 
concealed, in all probability, accord- 
ing to Schmidt, the terminal parts 
of the genital organs, just as the 
median part of the operculum in 
Phrynus and Thelyphonus conceals 
the complicated terminal portions 
of the genital organs. The posterior 
part of the operculum, like that of 
Phrynus and Thelyphonus, carried 
the first pair of branchiae, so Schmidt 
thinks from the evidence of markings 
on some specimens. 

Apparently an opercular ap- 
pendage of this kind is in reality 
the result of a fusion of the genital 
operculum with the first branchial appendage in forms such as the 
scorpion; for, in order that the tergal plates may correspond in 
number with the sternal in Eurypterus, etc., it is necessary to 
consider that the operculum is composed of two sternites joined 
together. Similarly in Thelyphonus, Phrynus, etc., tliis numerical 
correspondence is only observed if the operculum is looked upon 
as double. 

A restoration of the mesosomatic region of Eurypterus, viewed 




Fig. 78.— Eurypterus. 
The segments and appendages on the 
right are numbered in correspon- 
dence with the cranial system of 
lateral nerve-roots as found in verte- 
brates. M. 9 metastoma. The sur- 
face ornamentation is represented 
on the first segment posterior to the 
branchial segments. The opercular 
appendage is marked out by dots. 



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192 



THE ORIGIN OF VERTEBRATES 



Gen. duct. 



from the internal surface, might be represented by Fig. 79, in which 
the thick line represents the outline of the opercular segment, and 

the fainter lines the succeeding 
branchial segments. The middle 
and anterior part of the opercular 
segment carried the terminations 
of the genital organs; these I 
have represented, in accordance 
with our knowledge of the nature 
of these organs in the present-day 
scorpions, as a median elongated 
uterus, bilaterally formed, from 
which the genital ducts passed, 
probably as in Limulus, towards 
a mass of generative gland in the 
cephalic region, and not as in 
Scorpio or Thelyphonus, tailwards 
to the abdominal region. 

It is possible that in Holm's 
representation of Eurypterus, Fig. 
104, the genital duct on each side 
is indicated. 




Fig. 79. — Diagram to indicate the 
probable nature of the me8080- 
matic Segments of Eurypterus. 

The opercular segment is marked out by 
the thick black line. The segments 
I I. -VI. bear branchiae, and segment I. 
is supposed in the male to carry the 
uterus masculinus (Ut. Masc.) and 
the genital ducts. 



The Thyroid Gland of Ammoccetes. 

If we compare this mesosomatic region of Eurypterus with that 
of Ammoccetes, the resemblance is most striking, and gives a mean- 
ing to the facial nerve which is in absolute accordance with the 
interpretation already given of the glossopharyngeal and vagus 
nerves. In both cases the foremost respiratory or mesosomatic 
segment is double, the posterior lateral part alone bearing the 
branchue, while the median and anterior part bore in the one animal 
the uterus and genital ducts, in the other the thyroid gland and 
ciliated grooves. We are driven, therefore, to the conclusion that 
this extraordinary and unique organ, the so-called thyroid gland of 
Ammoccetes, which exists only in the larval condition and is got rid 
of as soon as the adult sexual organs are formed, shows the very form 
and position of the uterus of this invertebrate ancestor of Ammo- 
ccetes. What, then, is the nature of the thyroid gland in Ammoccetes ? 



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THE EVIDENCE OF THE THYROID GLAND 



1 93 



Throughout the vertebrate kingdom it is possible to compare the 
thyroid gland of one group of animals with that of another without 
coming across any very marked difference of structure right down 
to and including Petromyzon. When, however, we examine Ammo- 
ccetes, we find that the thyroid has 
suddenly become an organ of much 
more complicated structure, covering a 
much larger space, and bearing no re- 
semblance to the thyroid glands of the 
higher forms. At trans for maticn the 
thyroid of Ammoccetes is largely de- 
stroyed, and what remains of the gland 
in Petromyzon becomes limited to a few 
follicles resembling those of other fishes. 
The structure and position of this gland 
in Ammoccetes is so well known that it 
is unnecessary to describe it in detail. 
For the purpose, however, of making 
my points clear, I give in Fig. 80 the 
position and appearance of the thyroid 
gland (Th.) when the skin and under- 
lying laminated layer has been re- 
moved by the action of hypochlorite of 
soda. On the one side the ventral 
somatic muscles have been removed to 
show the branchial cartilaginous basket- 
work. 

The series of transverse sections in 
Fig. 81 represents the nature of the 
organ at different levels in front of and 
behind the opening into the respiratory 
chamber ; and in Fig. 82 I have sketched 
the appearance of the whole gland, 
viewed so as to show its opening 
into the respiratory chamber and its posterior curled-up termi- 
nation. 

The series of transverse sections (1-G, Fig. 81) show that we are 
dealing here with a central glandular chamber, C (Fig. 81 ((») and 
Fig. 82), which opens by the thyroid duct (Hi. 0.) into the pharyngeal 






Fig. 80. — Ventral View op 
Head Region of Ammoccetes. 

Th., thyroid gland; M. y lower 
lip, with its muscles. 



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194 



THE ORIGIN OF VERTEBRATES 




TKo. 



j> - Ls -V N ^ 




S 6 

Fig. 81.— Samples from a Complete Series of Transverse Sections through 

the Thyroid Gland of Ammoccetes. 

Sections 1 and 2 are anterior to the thyroid opening, Th. o.\ sections 3, 4, and 5 are 
through the thyroid opening ; and section 6 is posterior to the thyroid opening 
before the commencement of the curled portion. 



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THE EVIDENCE OF THE THYROID GLAND 1 95 

chamber, and is curled upon itself in its more posterior part. This 
central chamber divides, anteriorly to the thyroid orifice, into two * 
portions, A, A 1 (Fig. 82), giving origin to two tubes, B, B', which lie 
close alongside of, and extend further back than, the posterior limit 
of the curled portion of the central chamber, C. The structure of 
the central chamber, C, and, therefore, of the separate coils, is given 
in both Schneider's and Dohrn's pictures, and is represented in 
Fig. 81 (6), which shows the peculiar arrangement and character of 
the glandular cells typical of this organ, and also the nature of the 
central cavity, with the arrangement of the ciliated epitheliujn. The 
structure of each of the lateral tubes, B, is different from that of the 
central chamber, in that only half the central chamber is present 
in them, as is seen by the comparison of the tube B with the tube C 
in Fig. 81 (5 and 6), so that we may look upon the central chamber, 
C, as formed of two tubes, similar in structure to the tubes B, which 
have come together to form a single chamber by the partial absorp- 
tion of their walls, the remains of the wall being still visible as the 
septum, which partially divides the chamber, C, into halves. 

In the walls of each of these tubes is situated a continuous 
glandular line, the structure of the glandular elements being specially 
characterized by the length of the cells, by the large spherical nucleus 
situated at the very base of each cell, and by the way in which the 
cells form a wedge-shaped group, the thin points of all the wedge- 
shaped cells coming together so as to form a continuous line along 
the chamber wall. This free termination of the cells of the gland 
in the lumen of the chamber constitutes the whole method for the 
secretion of the gland ; there is no duct/no alveolus, nothing but this 
free termination of the cells. 

Moreover, sections through the portion A, A' (Fig. 82) show that 
here, as in the central chamber, C, four of these glandular lines open 
into a common chamber, but they are not the same four as in the case 
of the central chamber, for if we name these glandular lines on the left 
side a b, a V (Fig. 81), and on the right side c d, c' d', then the central 
chamber has opening into it the glands a b,c d, while the chambers of 
A and A' have opening into them respectively a b t a' b\ and c d, c' d'. 
Further, the same series of sections shows that the glands a and b are 
continuous with the glands a! and b' respectively across the apex of A, 
and similarly on the other side, so that the two glandular rows a b 
are continuous with the two glandular rows a' V, and we see that the 



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196 



THE ORIGIN OF VERTEBRATES 



cavity of the portion A or A' is formed by the bending over of the tube 
or horn, B or B', with the partial absorption of the septum so formed 
between the tube and its bent-over part. If, then, we uncoil the 
curled-up part of C, and separate the portion, B, on each side from the 
chamber, C, we see that the so-called thyroid of Ammocoptes may be 
represented as in Fig. 83, i.e. it consists of a long, common chamber, C, 

P.hr! 
Th. O .--v: :: " 




Fig. 82.— Diagrammatic Representation of the so-called Thyroid Gland op 

Ammocceteb. 

C, central chamber; A, A', anterior extremity; B, B\ posterior extremity; Th. o., 
thyroid opening into respiratory chamber; Ps. br., Ps. fcr\, ciliated grooves, 
Dohm's pseudo-branchial grooves. 



<== 




Fig. 83.— Thyroid Gland as it would appear if the Central Chamber were 
Uncurled and the Two Horns, D, fi\ separated from the Central 
Chamber. 

which, for reasons apparent afterwards, I will call the palceo-hysteron, 
which opens, by means of a large orifice, into the respiratory or 
pharyngeal chamber. The anterior end of this chamber terminates in 
two tubes, or horns, B, B', the structure of which shows that the median 
chamber, C, is the result of the amalgamation of two such tubes, and 
consequently in this chamber, or palcco-hystcron, the glandular lines 
(ire symmetrically situated on each side. 

Any explauation, then, of the thyroid gland of Ammocu*tes, must 



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THE EVIDENCE OF THE THYROID GLAND 1 97 

take into account the clear evidence that it is composed of two 
tubes, which have in part fused together to form an elongated central 
chamber, in part remain as horns to that chamber, and that in its 
walls there exist lines of gland-cells of a striking and characteristic 
nature. 

Further, this central chamber, with its horns, is not a closed 
chamber, but is in communication with the pharyngeal or respiratory 
chamber by three ways. In the first place, the central chamber, as 
is well known, opens into the respiratory chamber by a funnel-shaped 
opening — the so-called thyroid duct (Th. 0.). In the second place, 
there exist two ciliated grooves (Ps. br. f Ps. br'.), the pseudo-branchial 
grooves of Dohrn, which have direct communication with the thyroid 
chamber. The manner in which these grooves communicate with the 
thyroid chamber has never, to my knowledge, been described pre- 
viously to my description in the Journal of Physiology and Anatomy ; 
it is very instructive, for, as I have there shown, each groove enters 
into the corresponding lateral horn, so that, in reality, there are three 
openings into the thyroid chamber or palaeo-hysteron — a median 
opening into the central chamber, and a separate opening into each 
lateral horn. 

The system of ciliated grooves on the inner ventral surface of 
the respiratory chamber of Ammoccetes was originally described by 
Schneider as consisting of a single median groove, which extends 
from the opening of the thyroid to the posterior extremity of the 
branchial chamber, and a pair of grooves, or semi-canals, which, 
starting from the region of the thyroid orifice, run headwards and 
diverge from each other, becoming more and more lateral, and more 
and more dorsal, till they come together in the mid-dorsal pharyngeal 
line below the auditory capsules. The latter are the pseudo-branchial 
grooves of Dohrn, of which I have already spoken. Schneider 
looked upon the whole of this system as a single system, for he 
speaks of " a ciliated groove, which extends from the orifice of the 
stomach (i.e. anterior intestine) to the orifice of the thyroid, then 
divides into two, and runs forward right and left of the median ridge, 
etc. ,, Dohrn rightly separates the median ciliated groove posterior 
to the thyroid orifice (seen in Fig. 81 (G)) from the paired pseudo- 
branchial grooves ; the former is a shallow depression which opens 
into the rim of the thyroid orifice, while the latter has a much more 
intimate connection with the thyroid gland itself. 



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s~ 



198 THE ORIGIN OF VERTEBRATES 

A series of sections, such as is given in Fig. 81, -shows the relation 
of this pair of ciliated grooves to the thyroid better than any elaborate 
description. In the first place, it is clear that they remain separate 
up to their termination — they do not join in the middle line to open 
into the thyroid duct ; in the second place, they are separate from 
the thyroid orifice — they do not terminate at the rim of the orifice, 
as is the case with the median groove just mentioned, but continue 
on each side on the wall of the thyroid duct (Fig. 81 (2)), gradually 
moving further and further away from the actual opening of the duct 
into the pharyngeal chamber. During the whole of their course on 
the wall of the funnel-shaped duct they retain the character of 
grooves, and are therefore open to the lumen of the duct. The direc- 
tion of the groove (Ps. br.) shifts as it passes deeper and deeper 
towards the thyroid, until at last, as seen in Fig. 81 (3 and 4), it is 
continuous with the narrow diverticulum of the turned-down single 
part of the thyroid (B), or turned-down horn, as I have called it. 
In other words, the median chamber opens into the pharyngeal or 
respiratory chamber by a single large, funnel-shaped opening, and, in 
addition, the two ciliated grooves terminate in the lateral horns on 
each side, and only indirectly into the central chamber, owing to their 
being semi-canals, and not complete canals. If they were originally 
canals, and not grooves, then the thyroid of Ammoccetes would be 
derived from an organ composed of a large, common glandular 
chamber, which opened into the respiratory chamber by means of an 
extensive median orifice, and possessed anteriorly two horns, from 
each of which a canal or duct passed headwards to terminate some- 
where in the region of the auditory capsule. 

Dohrn has pointed out that a somewhat similar structure and 
topographical arrangement is found in Amphioxus and the Tunicata, 
the gland-cells being here arranged along the hypobranchial groove 
to form the endostyle and not shut off to form a closed organ, as in 
the thyroid of Ammoccetes. Dohrn concludes, in my opinion rightly, 
that the endostyle in the Tunicata and in Amphioxus represents the 
remnants of the more elaborate organ in Ammoccetes, and that, 
therefore, in order to explain the meaning of these organs in the 
former animals, we must first find out their meaning in Ammoccetes. 
Dohrn, however, goes further than this; for just as he considers 
Amphioxus and the Tunicata to have arisen by degeneration from an 
Ammocoetes-like form, so he considers Ammoccetes to have arisen 



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THE EVIDENCE OF THE THYROID GLAND I99 

from a degenerated Selachian ; therefore, in order to be logical, he 
ought to show that the thyroid of Ammoccetes is an intermediate down- 
ward step between the thyroid of Selachians and that of Amphioxus 
and the Tunicates. Here, it seems to me, his argument utterly breaks 
down ; it is so clear that the thyroid of Petromyzon links on to that 
of the higher fishes, and that the Ammoccetes thyroid is so immeasur- 
ably more complicated and elaborate a structure than is that of 
Petromyzon, as to make it impossible to believe that the Ammoccetes 
thyroid has been derived by a process of degeneration from that of 
the Selachian. On the contrary, the manner in which it is eaten up 
at transformation and absolutely disappears in its original form is, 
like the other instances mentioned, strong evidence that we are 
dealing here with an ancestral organ, which is confined to the larval 
form, and disappears when the change to the higher adult condition 
takes place. Dohrn's evidence, then, points strongly to the conclu- 
sion that the starting-point of the thyroid gland in the vertebrate 
series is to be found in the thyroid of Ammocoetes, which has given 
rise, on the one hand, to the endostyle of Amphioxus and the Tuni- 
cata, and on the other, to the thyroid gland of Petromyzon and the 
rest of the Vertebrata. 

The evidence which I have just given of the intimate connection 
of the two pseudo-branchial grooves with the thyroid chamber shows, 
to my mind, clearly that Dohrn is right in supposing that morpho- 
logically these two grooves and the thyroid must be considered 
together. His explanation is that the whole system represents a 
modified pair of branchial segments distinct from those belonging to 
the Vllth and IXth nerves. The cavity of the thyroid and the 
pseudo-branchial grooves are, therefore, according to him, the remains 
of the gill-pouches of this fused pair of branchial segments, which no 
longer open to the surface, and the glandular tissue of the thyroid is 
derived from the modified gill-epithelium. This view of Dohrn's, 
which he has urged most strongly in various papers, is, I think, 
right in so far as the separateness of the thyroid segment is con- 
cerned, but is not right, and is not proven, in so far as concerns the 
view that the thyroid gland is a modified pair of gills. 

We may distinctly, on my view, look upon the thyroid segment, 
with its ciliated grooves and its covering plate of muco-cartilage, as 
a distinct paired segment, homologous with the branchial segments, 
without any necessity of deriving the thyroid gland from a pair of gills, 



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200 



THE ORIGIN OF VERTEBRATES 



The evidence that such a median segment has been interpolated 
ventrally between the foremost pairs of branchial segments is 
remarkably clear, for the limits ventrally of the branchial segments 
are marked out on each side by the ventral border of the carti- 
laginous basket-work ; and it is well known, as seen in Fig. 80, that 
whereas this cartilaginous framework on the two sides meets together 
in the middle ventral line in the posterior branchial region, it diverges 
in the anterior region so as to form a tongue-shaped space between 




&L»t.VII«X 



Fig. 84.— Diagram of (A) Ventral Surface and (B) Lateral Surface of Ammo- 

COETES, SHOWING THE ARRANGEMENT OF THE EPITHELIAL PlTS ON THE BRAN- 
CHIAL Region, and their innervation by VII., the Facial, IX., the 
Glossopharyngeal, and X l -X* t the Vagus Nerves. 

the branchial segments on the two sides. This space is covered over 
with a plate of muco-cartilage which bears on its inner surface the 
thyroid gland. 

In addition to this evidence that we are dealing here with a 
ventral tongue-like segment belonging to the facial nerve which is 
interpolated between the foremost branchial segments, we find the 
most striking fact that at transformation the whole of this muco- 
cartilaginous plate disappears, the remarkable thyroid gland of the 



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THE EVIDENCE OF THE THYROID GLAND 



20I 



Ammoccetes is eaten up, and nothing is left except a small, totally 
different glandular mass; and now the cartilaginous basket-work 
meets together in the middle line in this region as well as in the 
more posterior region. In other words, the striking characteristic 




8 X 5 



9 X 6 



Fig. 85. — Facial Segment of Ammoccetes marked out by Shadikg. 

VII. 1, thyroid part of segment ; VII. 2, hyoid or branchial part ; 8-9, succeeding 
branchial segments belonging to IXth and Xth nerves ; K, the velar folds ; 
Ps. br n Dohrn's pseudo-branchial groove; Th. v., thyroid opening; C, curled 
portion of thyroid. 

of transformation here is the destruction of this interpolated seg- 
ment, and the resulting necessary drawing together ventrally of 
the branchial segments on each side. 

Moreover, another most instructive piece of evidence pointing in 
the same direction is afforded by the behaviour of the ventral epithelial 



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202 THE ORIGIN OF VERTEBRATES 

pits, as determined by Miss Alcock. Although there is no indication 
on the ventral surface of the skin of any difference between the 
anterior and posterior portions of the respiratory region, yet when 
the ventral rows of the epithelial pits supplied by each branchial 
nerve are mapped out, we see how the most anterior ones diverge 
more and more from the mid-ventral line, following out exactly the 
limits of the underlying muco-cartilaginous thyroid plate (Fig. 84). 

The whole evidence strongly leads to the conclusion that the 
thyroid portion of the facial segment was inserted as a median tongue 
between the foremost branchial segments on each side, and that, 
therefore, the whole facial segment, consisting as it does of a thyroid 
part and a hyoid or branchial part, may be represented as in Fig. 
85, which is obtained by splitting an Ammocoetes longitudinally 
along the mid-dorsal line, so as to open out the pharyngeal chamber 
and expose the whole internal surface. The facial segment is marked 
out by shading lines, the glossopharyngeal and vagus segments and 
the last of the trigeminal segments being indicated faintly. The 
position of the thyroid gland is indicated by oblique lines, C being 
the curled portion. 

The Uterus of the Scorpion Group. 

Seeing how striking is the arrangement and the structure of the 
glandular tissue of this thyroid, how large the organ is and how 
absolutely it is confined to Ammocoetes, disappearing entirely as 
such at transformation, we may feel perfectly certain that a corre- 
sponding, probably very similar, organ existed in the invertebrate 
ancestor of the vertebrate; for the transformation process consists 
essentially of the discarding of invertebrate characteristics and the 
putting on of more vertebrate characters ; also, so elaborate an organ 
cannot possibly have been evolved as a larval adaptation during the 
life of Ammocoetes. We may therefore assert with considerable con- 
fidence that the thyroid gland was the palcco-hysteron, and was 
derived from the uterus of the ancient palaeostracau forms. If, then, 
it be found that a glandular organ of this very peculiar structure and 
arrangement is characteristic of the uterus of any living member of 
the scorpion group, then the confidence of this assertion is greatly 
increased. 

In Liniulus, as already stated, the genital ducts open separately 



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THE EVIDENCE OF THE THYROID GLAND 203 

on each side of the operculum, and do not combine to form a 
uterus ; I have examined them and was unable to find any glandular 
structure at all resembling that of the thyroid gland of Ammoccetes. 
I then turned my attention to the organs of the scorpion, in which 
the two ducts have fused to form a single uterus. 

I there found that both in the male and in the female the genital 




Fio. 86.— Section thbough the Terminal Chamber or Uterus of the Male 

Scorpion. 

C, cavity of chamber. A portion of the epithelial lining of the channels of emission 
is drawn above the section of the uterus. 

ducts on each side terminate in a common chamber or uterus, which 
underlies the whole length of the operculum, and opens to the 
exterior in the middle line, as shown in Fig. 76. In transverse 
section, this uterus has the appearance shown in Fig. 86, i.e. it is 
a large tube, evidently expansible, lined with a chitinous layer and 
epithelial cells belonging to the chitinogenous layer, except in two 
symmetrical places, where the uniformity of the uterine wall is 



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204 



THE ORIGIN OF VERTEBRATES 



interrupted by two large, remarkable glandular structures. The 
structure of these glands is better shown by means of sagittal sec- 
tions. They are composed of very long, wedge-shaped cells, each of 
which possesses a large, round nucleus at the basal end of the cell 
(Fig. 87). These cells are arranged in bundles of about eight to ten, 
which are separated from each other by connective tissue, the apex 
of each conical bundle being directed into the cavity of the uterus ; 
where this brush -like termination of the cells reaches the surface, the 
chitinous layer is absent, so that this layer is, on surface view, seen 



>» 




Fig. 87.— Longitudinal Sec- 
tion THROUGH THBEB OF 

the Cones op the Uterine 
Glands of the Scorpion. 











Fig. 88.— Sagittal Section through 
the Uterine Gland of Scorpion, 
showing the Internal Chitinous 
Surface (b) and the Glandular 
Cones (a) cut through at various 
distances from the Internal Sur- 
face. 



(Fig. 88 (b)) to be pitted with round holes over that part of the 
internal surface of the uterus where these glands are situated. Each 
of these holes represents the termination of one of these cone-shaped 
wedges of cells. If the section is cut across at right angles to the 
axis of these cones, then its appearance is represented in Fig. 88 (a), 
and shows well the arrangement of the blocks of cells, separated from 
each other by connective tissue. When the section passes through 
the basal part of the cones, and only in that case, then the nuclei 
of the cells appear, often in considerable numbers in one section, as 



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THE EVIDENCE OF THE THYROID GLAND 



205 




is seen in Fig. 89. In Fig. 88 the section shows at b the holes in 
the chit in in which the cones terminate, and then a series of layers 
of sections through the cones further and 
further away from their apices. 

These conical groups of long cells, repre- 
sented in Fig. 87, form on each side of the 
uterus a gland, which is continuous along 
its whole length, and thus formR a line 
of secreting surface on each side, just as 
in the corresponding arrangement of the 
glandular structures in the thyroid of Am- 
moccetes. This uterus and glandular ar- 
rangement is found in both sexes ; the gland is, however, more 
developed in the male than in the female scorpion. 

The resemblance between the structure of the thyroid of Ammo- 
cartes and the uterus of the scorpion is most striking, except in two 
respects, viz. the nature; of the lining of the non-glandular part of 
the cavity — in the one case ciliated, in the other chitinous — and the 
place of exit of the cavity, the thyroid of Ammoccetes opening into 



Fig. 89.— Transverse Sec- 
tion THROUGH THE BASAL 

Part op the Uterine 
Glands op the Scorpion. 



AMMOCCETE& 



8CORPION. 




Muco-cartilage 

Branchial cartilage 



Operculum 



Fig. 90.— Section of Central Chamber op Thyroid of Ammoccetes and Section 
of Uterub op Scorpion. 

the respiratory chamber, while the uterus of Scorpio opens direct to 
the exterior. 

With respect to the first difference, the same difficulty is met 



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206 THE ORIGIN OF VERTEBRATES 

with in the comparison of the ciliated lining of the tube in the 
central nervous system of vertebrates with the chitinous lining 
of the intestine in the arthropod. Such a difference does not seem 
to me either unlikely or unreasonable, seeing that cilia are found 
instead of chitin in the intestine of the primitive arthropod Peri- 
patus. Also the worm- like ancestors of the arthropods almost 
certainly possessed a ciliated intestine. Finally, the researches of 
Hardy and McDougall on the intestine of Daphnia point directly to 
the presence of a ciliated rather than a chitinous epithelial lining of 
the intestine in this animal — all evidence pointing to the probability 
that in the ancient arthropod forms, derived as they were from the 
annelids, the intestine was originally ciliated and not chitinous. It 
is from such forms that I suppose vertebrates to have sprung, and 
not from forms like the living king-crabs, scorpions, Apus, Bran- 
chipus, etc. I only use them as illustrations, because they are the 
only living representatives of the great archaic group, from which 
the Crustacea, Arachnida, and Vertebrata all took origin. 

The second difference is more important, and is at first sight 
fatal to any comparison between the two organs. How is it possible 
to compare the uterus of the scorpion, which opens on the surface by 
an external genital opening, with the thyroid of Ammocoetes, which 
opens by an internal opening into the respiratory chamber ? However 
close may be the histological resemblance of structure in the two 
cases, surely such a difference is too great to be accounted for. 

It is, however, to be remembered that the operculum of Scorpio 
covers only the terminal genital apparatus, and does not, therefore, 
resemble the operculum of the presumed ancestor of Ammocoetes, 
which, as already argued, must have resembled the operculum of 
Thelyphonus with its conjoint branchial and genital apparatus, 
rather than that of Scorpio. Before, therefore, making too sure of 
the insuperable character of this difficulty, we must examine the 
uterus of the Pedipalpi, and see the nature of its opening. 

The nature of the terminal genital organs in Thelyphonus has 
been described to some extent by Blanchard, and more recently by 
Tarnani. The ducts of the generative organs terminate, according to 
the latter observer, in the large uterus, which is found both in the 
male and female ; he describes the walls of the uterus in the female 
as formed of elongated glandular epithelium, with a strongly- 
developed porous, chitinized intiuia. In the male, he says that the 



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THE EVIDENCE OF THE THYROID GLAND 



207 



epithelium of the uterus masculinus and its processes is extraordi- 
narily elongated, the chitin covering being thick. In these animals, 
then, the common chamber or uterus into which the genital ducts 
empty, which, like the corresponding chamber in the scorpion, 
occupies the middle region of the operculum, is a large and con- 
spicuous organ. Further, and this is a most striking fact, the 
uterus masculinus does not open direct to the exterior, but into the 
genital cavity, "which lies above the uterus, so that the latter is 
situated between the lower wall of the genital cavity and the outer 
integument." The opening, 
therefore, of the uterus is not 
external but internal, into the 
large internal space known 
as the genital cavity. The 
arrangement is shown in Fig. 
91, taken from Tarnani's 
paper, which represents a Cen.CK 
diagrammatic sagittal section 
through the exit 6f the male 
genital duct. Yet another 
most striking fact is described 
by Tarnani. This genital 
cavity is continuous with the 

pulmonary or gill cavities on Thethick lineig the operculum, composed of 
each side, SO that instead of a two segments, I. and II. Ut. Masc, uterus 
single opening for the genital masc^us ; Oen. Ch., genital chamber ; Int. 
°. / ,° i-i PP» internal opening; Ext. Op., external 

products and one on each Side opening common to the genital and respira- 

for each gill-pouch, as would tory organs. 

be the case if the arrangement 

was of the same kind as in the scorpion, there is a single large 

chamber, the genital chamber, common to both respiratory and 

genital organs. 

This genital chamber, according to Tarnani, opens to the exterior 
by a single median opening between the operculum and the succeed- 
ing segment ; similarly, a communication from side to side exists 
between the second pair of gill-pouches. I have been able to 
examine Hypoctonus formosus and Thelyphonus caudatus, and in both 
cases, in both male and female, the opening to the exterior of the 
common chamber for respiration and for the genital products was 




Eact.Op. 
-Ill 



Fig. 91. — Sagittal Median Diagrammatic 
Section through the Operculum op the 
Male Thelyphonus. (From Tarnanl) 



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208 THE ORIGIN OF VERTEBRATES 

not a single opening, as described by Tarnani in Thelyphonus aspe- 
ratns, but on each side of the middle line, a round orifice closed by a 
lid, like the nest of the trapdoor spider, led into the common genital 
chamber (Gen. Ch.) into which both uterus and gills opened. In 
Fig. 77 I have endeavoured to represent the arrangement of the 
genital and respiratory organs in the male Thelyphonus according to 
Tarnani's and my own observations. 

If we may take Thelyphonus as a sample of the arrangement in those 
scorpions in which the operculum was fused with the first branchial 
appendage, among which must be included the old sea-scorpions, then 
it is most significant that their uterus should open internally into a 
cavity which was continuous with the respiratory cavity. Thus not 
only the structure of the gland, but also the arrangement of the internal 
opening into the respiratory, or, as it became later, the pharyngeal 
cavity, is in accordance with the suggestion that the thyroid of Ammo- 
crotes represents the uterus of the extinct Eurypterus-like ancestor. 

Into this uterus the products of the generative organs were poured 
by ineans of the vasa dcfercntia, so that there was not a single 
median opening or duct in connection with it, 'but also two side 
openings, the terminations of the vasa defcrentia. These are described 
by Tarnani in Thelyphonus as opening into the two horns of the 
uterus, which thus shows its bilateral character, although the body 
of the organ is median and single ; these ducts then pass within the 
body of the animal, dorsal to the uterus, towards the testes or ovaries 
as the case may be, organs which are situated in these animals, as in 
other scorpions, in the abdomen, so that the direction of the ducts 
from the generative glands to the uterus is headwards. If, however, 
we examine the condition of affairs in Limulus, we find that the 
main mass of the generative material is cephalic, forming with the 
liver that dense glandular mass which is packed round the supra- 
a>sophageal and prosomatic ganglia, and round the stomach and 
muscles of the head-region. From this cephalic region the duct 
passes out on each side at the junction of the prosomatic and meso- 
somatic carapace to open separately on the posterior surface of the 
operculum, near the middle line, as is indicated in Fig. 75. 

We have, therefore, two distinct possible positions for the genital 
ducts among the group of extinct scorpion-like animals, the one 
from the cephalic region to the operculum, and the other from the 
abdominal region to the operculum. 



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THE EVIDENCE OF THE THYROID GLAND 209 

The Generative Glands ok Limulus and its Allies. 

The whole argument, so far, has in every case ended with the 
conclusion that the original scorpion-like form with which I have 
been comparing Ammocoetes resembled in many respects Limulus 
rather than the present-day scorpions, and therefore in the case also 
of the generative organs, with which the thyroid gland or palaeo- 
hysteron was in connection, it is more probable that they were 
cephalic in position rather than abdominal. If this were so, then 
the duct on each side, starting from the median ventral uterus, would 
take a lateral and dorsal course to reach the huge mass of generative 
gland lying within the prosomatic carapace, just as I have repre- 
sented in the figure of Eurypterus (Fig. 79), a course which would 
take much the same direction as the ciliated groove in Ammoccetes. 

We ought, therefore, on this supposition, to expect to find the 
remains of the invertebrate generative tissue, the ducts of which 
terminated in the thyroid, in the head-region, and not in the 
abdomen. 

Upon removal of the prosomatic carapace of Limulus, a large 
brownish glandular-looking mass is seen, in which, if it happens to 
be a female, masses of ova are very conspicuous. This mass is com- 
posed of two separate glands, the generative glands and the hepatico- 
pancreatic glands — the so-called liver — and surrounds closely the 
central nervous system and the alimentary canal. From the genera- 
tive glands proceed the genital ducts to terminate on the posterior 
surface of the operculum. From the liver ducts pass to the pyloric 
end of the cephalic stomach, and carry the fluid by means of whicli 
the food is digested, for, in all these animals, the active digesting 
juices are formed in the so-called liver, and not in the cells of the 
stomach or intestine. 

It is a very striking fact that the brain of Ammoctetes is much 
too small for the brain-case, and that the space between brain and 
brain-case is filled up with a very peculiar glandular-looking tissue, 
which is found in Ammoccetes and not elsewhere. Further, it is also 
striking that in the brain of Ammocoetes there should still exist the 
remains of a tube extending from the IVth ventricle to the surface at 
the conus iiost-commissuralis, which can actually be traced right into 
this tissue on the outside of the brain (see Fig. 13, a-c, PI. XXVL, 
in my paper in the Quarterly Journal of Microscopical Science). 



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2IO THE ORIGIN OF VERTEBRATES 

This, in my opinion, is the last remnant of one of the old liver-ducts 
which extended from the original stomach and intestine into the 
cephalic liver-mass. This glandular-looking material is shown 
surrounding the pineal eye and its nerve, in Fig. 31, also in 
Fig. 22, and separately in Fig. 92. It is composed of large cells, 
with a badly staining nucleus, closely packed together with lines 
of pigment here and there between the cells ; this pigment is 
especially congregated at the spot where the so-called liver-duct 
loses itself in this tissue. The protoplasm in these large cells does 
not stain well, and with osmic acid gives no sign of fat, so that 

Ahlborn's description of this tissue as a 
peculiar arachnoideal fat -tissue is not 
true; peculiar it certainly is, but fatty 
it is not. 

This tissue has been largely de- 
scribed as a peculiar kind of connective 
tissue, which is there as packing mate- 
rial, for the purpose of steadying a brain 
too small for its case. On the face of 
Fig7 92. -Drawing op the ifc such an explanation is unscientific; 
Tissue which surrounds certainly for all those who really believe 
the Brain of Ammocosteb. i n evolution, it is out of the question 

to suppose that a brain-case has been 
laid down in the first instance too large for the brain, in order 
to provide room for a subsequent increase of brain; just as it is 
out of the question to suppose that the nervous system was laid 
down originally as an epithelial tube in order to provide for the 
further development of the nervous system by the conversion of 
more and more of that tube into nervous matter. Yet this latter 
proposition has been seriously put forward by professed believers in 
evolution and in natural selection. 

This tissue bears no resemblance whatever to any form of con- 
nective tissue, either fatty or otherwise. By every test this tissue 
tells as plainly as possible that it is a vestige of some former 
organ, presumably glandular, which existed in that position; that 
it is not there as packing material because the brain happened 
to be too small for its case, but that, on the contrary, the brain 
is too small for its case, because the case, when it was formed, 
included this organ as well as the brain ; in other words, this tissue 




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THE EVIDENCE OF THE THYROID GLAND 211 

is there because it is the remnant of the great glandular mass which 
so closely surrounds the brain and alimentary canal in animals such 
as Limulus. In my paper in the Quarterly Journal of Microscopical 
Science, in which I was comparing the tube of the vertebrate nervous 
system with the alimentary canal of the invertebrate, I spoke of this 
tissue as being the remnant of the invertebrate liver. At the same 
time the whole point of my argument was that the glandular material 
surrounding the brain of Limulus was made up of two glands — liver 
and generative gland — so that this tissue might be the remnant of 
either one or the other, or both. All I desired, at that time, was 
to point out the glandular appearance of this so-called packing tissue, 
which surrounded the brain-region of Ammoccetes, in connection with 
the fact that the brain and alimentary canal of Limulus were closely 
surrounded with a glandular mass composed partly of liver, partly of 
the generative gland. At present, I think these large cells found 
round the brain in Ammoccetes are much more likely to be the 
remnant of the generative gland than of the liver ; the size of the 
cells and their arrangement recalls Owen's picture of the generative 
gland in Limulus, and seeing how important all generative glands 
are in their capacity of internal secreting glands, apart entirely from 
the extrusion of the ripe generative products, and how unimportant 
is an hepato-pancreas when the alimentary canal is closed, it is much 
more likely that of the two glands the former would persist longer 
than the latter. It may be that all that is left of the old hepato- 
pancreas consists of the pigment so markedly found in between these 
cells, especially at the place where the old liver-duct reaches the 
surface of the brain ; just as the only remnant of the two pineal eyes 
in the higher vertebrates is the remains of the pigment, known as 
brain-sand, which still exists in the pineal gland of even the highest 
vertebrate. This, however, is a mere speculation of no importance. 
What is important is the recognition of this tissue round the brain 
as the remnant of the glandular mass round the brain of animals such 
as Limulus. Still further confirmation of the truth of this comparison 
will be given when the origin of the auditory organ comes up for 
discussion. 

I conclude, therefore, from the evidence of Ammoccetes, that the 
generative glands in the ancestral form were situated largely in the 
cephalic region, and suggest that the course and direction of the ciliated 
pseudo-branchial grooves on each side indicate the direction of the 



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212 THE ORIGIN OF VERTEBRATES 

original opercular ducts by which the generative products were con- 
veyed to the uterine chamber, i.e. to the chamber of the thyroid 
gland, and thence to the common genital and respiratory cavity, and 
so to the exterior. 

It is easy to picture the sequence of events. First, the generative 
glands, chiefly confined to the cephalic region, communicating with 
the exterior by separate ducts on the inner surface of the operculum 
as in Limulus. Then, in connection with the viviparous habit, these 
two oviducts fused together to form a single chamber, covered by the 
operculum, which opened out to the exterior by a single opening as 
in Scorpio : or, in forms such as Eurypterus, in which the operculum 
had amalgamated with the first branchial appendage and possessed a 
long, tongue-like ventral projection, the amalgamated ducts formed 
a long uterine chamber wliich opened internally into the genital 
chamber — a chamber which, as in Thelyphonus, was common with 
that of the two gill-chambers, while at the same time the genital ducts 
from the cephalic generative material opened into two uterine horns 
which arose from the anterior part of the uterus, as in Thelyphonus. 

Such an arrangement would lead directly to the condition found 
in Ammoccetes, if the generative material around the brain lost its 
function, owing to a new exit for generative products being formed 
in the posterior part of the body. The connection of the genital duct 
with this cephalic gland being then closed and cut off by the brain- 
case, the position of the oviducts would still be shown by the ciliated 
grooves opening into the folded-down thyroid tube, i.e. the folded- 
down horns of the uterus; the uterus itself would remain as the 
main body of the thyroid and still open by a conspicuous orifice into 
the common respiratory chamber. Next, in the degeneration process, 
we may suppose that not only the oviducts opened out to form the 
ciliated groove, but that the uterine chamber itself also opened out, 
and thus formed the endostyle of Amphioxus and of the Tunicata. 

It might seem at first sight improbable that a closed tube should 
become an open groove, although the reverse phenomenon is common 
enough ; the difficulty, however, is clearly not considered great, for it 
is precisely what Dohrn imagines to have taken place in the conversion 
of the thyroid of Ammoccetes into the endostyle of Amphioxus and 
the Tunicata ; it is only carrying on the same idea a stage further to 
see in the open, ciliated groove of Ammoccetes the remains of the 
closed genital duct of Limulus and its allies. 



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THE EVIDENCE OF THE THYROID GLAND 213 

Such is the conclusion to which the study of the thyroid gland 
in Ammoccetes seems to me to lead, and one cannot help wondering 
why such an unused and rudimentary organ should have remained 
after its original function had gone. Is it possible to find out its 
function in Ammoccetes ? 



The Function of the Thyroid Gland in Ammoccetes. 

The thyroid gland has been supposed to secrete mucus into the 
respiratory chamber for the purpose of entangling the particles of food, 
and so aiding in digestion. I see no sign of any such function ; neither 
by the thionin method, nor by any other test, have Miss Alcock and 
myself ever been able to see any trace of mucous secretion in the thy- 
roid, and, indeed, the thyroid duct is always remarkably free from any 
sign of any secretion whatever. Not only is there no evidence of any 
mucous secretion in the thyroid of the fully developed Ammoccetes, 
but also no necessity for such secretion from Dohrn's point of view, 
for so copious a supply of mucus is poured out by the glands of the 
branchiae, along the whole pharyngeal tract, especially from the cells 
of the foremost or hyoid gills, as to mix up with the food as 
thoroughly as can possibly be needed. Further, too, the ciliated 
pharyngeal bands described by Schneider are amply sufficient to 
move this mixed mass along in the way required by Dohrn. Finally, 
the evidence given by Miss Alcock is absolutely against the view that 
the thyroid takes any part in the process of digestion, while, on the 
other hand, her evidence directly favours the view that these 
glandular branchial mucus-secreting cells play a most important part 
in the digestive process. 

In Fig. 93, A is a representation of the respiratory tissue of a 
normal gill ; B is the corresponding portion of the first or hyoid gill, 
in which, as is seen, the whole of the respiratory epithelium is 
converted into gland-tissue of the nature of mucous cells. 

To sum up, the evidence is clear and conclusive that the Ammo- 
ccetes possesses in its pharyngeal chamber mucus-secreting, glands, 
which take an active part in the digestive process, which do not in 
the least resemble either in structure or arrangement the remarkable 
cells of the thyroid gland, and that the experimental evidence that 
the latter cells either secrete mucus or take any part in digestion 
is so far absolutely negative. It is, of course, possible, that they 



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214 



THE ORIGIN OF VERTEBRATES 



may contain mucin in the younger developmental stages, and there- 
fore possible that they might at that stage secrete it ; they certainly, 
however, show no sign of doing so in their more adult condition, and 
cannot be compared in the very faintest degree to the glandular cells 
of the pharyngeal region. It is also perfectly possible for gland-cells 
belonging to a retrograde organ to become mucus-secreting, and so to 

give rise to the cells of Am- 
phioxus and the Tunicata. 

If, then, these cells were 
not retained for digestive 
purposes, what was their 
function? To answer this 
question we must first know 
the function of the corre- 
sponding gland-cells in the 





uterus of the scorpion, which 
undoubtedly secreted into 
the cavity of the uterus and 
took some part in connection 
with the generative act, and 
certainly not with digestion. 
What the function of these 
cells is or in what way they 
act I am unable at present 
to say. I can only suppose 
that the reason why the 
thyroid gland has persisted 
throughout the vertebrate 
kingdom, after the genera- 
tive tissues had found a new 
outlet for their products in the body-cavity of the posterior region, 
is because it possessed some important function in addition to that 
connected with the exit of the products of the generative organs ; a 
function which was essential to the well-being, or even to the life of 
the animal. We do not know its function in the scorpion, or the 
nature of its secretion in that animal. We know only that physiology 
at the present day has demonstrated clearly that the actual external 
secretion of a gland may be by no means its most important function ; 
in addition, glands possess what is called an internal secretion, viz. a 



Fig. 93.— A, Portion op a Gill op Ammo- 

CCETE8 WITH ORDINARY RESPIRATORY EPI- 
THELIUM J B, Corresponding Portion of 
the First or Hyoid Gill. 



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THE EVIDENCE OF THE THYROID GLAND 21 5 

secretion into the blood and lymph, and this latter secretion may be 
of the most vital importance. Now, the striking fact forces itself 
prominently forward, that the thyroid gland of the higher vertebrates 
is the most conspicuous example of the importance of such internal 
secretion. Here, although ductless, we have a gland which cannot 
be removed without fatal consequences. Here, in the importance of 
its internal secretion, we have a reason for the continued existence 
of this organ ; an organ whicli remains much the same throughout 
the Vertebrata down to and including Petromyzon, but, as is seen 
at transformation, is all that remains of the more elaborate, more 
extensive organ of Ammoccetes. Surely we may argue that it is 
this second function which has led to the persistence of the thyroid, 
and that its original form, without its original function, is seen in 
Ammoccetes, because that is a larval form, and not a fully-developed 
animal. As soon as the generative organs of Petromyzon are developed 
at transformation, all trace of its connection with a genital duct 
vanishes, and presumably its internal secretory function alone remains. 

Yet, strange to say, a mysterious connection continues to exist 
between the thyroid gland and the generative organs, even up to 
the highest vertebrate. That the thyroid gland, situated as it is 
in the neck, should have any sympathy with sexual functions if it 
was originally a gland concerned with digestion is, to say the least 
of it, extremely unlikely, but, on the contrary, likely enough if it 
originated from a glandular organ in connection with the sexual 
organs of the palseostracan ancestor of the vertebrate. 

Freund has shown, and shown conclusively, that there is an 
intimate connection between the condition of the thyroid gland and 
the state of the sexual organs, not only in human beings, but also 
in numerous animals, such as dogs, sheep, goats, pigs, and deer. He 
points out that the swelling of the gland, whicli occurs in consequence 
of sexual excitement (a fact mentioned both in folk-lore tales and in 
poetical literature), and also the swelling at the time of puberty, may 
both lead to a true goitrous enlargement ; that most of the permanent 
goitres commence during a menstrual period ; that during pregnancy 
swelling of the thyroid is almost universal, and may become so ex- 
treme as to threaten suffocation, or even cause death ; that the period 
of puberty and the climacteric period are the two maximal periods 
for the onset of goitre, and that exophthalmic goitre especially is 
associated with a special disease connected with the uterus. 



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216 THE ORIGIN OF VERTEBRATES 



Summary. 

Step by step in the preceding chapters the evidence is accumulating" in 
favour of the origin of vertebrates from a member of the palfeostracan group. 
In a continuously complete and harmonious manner the evidence has throughout 
been most convincing when the vertebrate chosen for the purpose of my argu- 
ments has been Ammocoetes. 

So many fixed points have been firmly established as to enable us to proceed 
further with very great confidence, in the full expectation of being able 
ultimately to homologize the Vertebrata with the Palaeostraca even to minute 
details. 

Perhaps the most striking and unexpected result of such a comparison is the 
discovery that the thyroid gland is derived from the uterus of the palfeostracan 
ancestor. Yet so clear is the avidence that it is difficult to see how the homology 
can be denied. 

In the one animal (Palaeostraca) the foremost pair of mesosomatic appendages 
forms the operculum, which always bears the terminal generative organs and 
is fused in the middle line. In many forms, essentially in Eurypterus and the 
ancient sea-scorpions, the operculum was composed of two segments fused 
together : an anterior one which carried the uterus, and a posterior one which 
carried the first pair of branchiae. 

In the other animal (Ammocoetes) the foremost segments of the mesosomatic 
or respiratory region, immediately in front of the glossopharyngeal segments, 
are supplied by the facial nerve, and are markedly different from those supplied 
by the vagus and glossopharyngeal, for the facial supplies two segments fused 
together; the anterior one, the thyroid segment, carrying the thyroid gland, 
the posterior one, the hyoid segment, carrying the first pair of branchiae. 

Just as in Eurypterus the fused segment, carrying the uterus on its internal 
surface, forms a long median tongue which separates the most anterior branchial 
segments on each side, so also the fused segment carrying the thyroid forms in 
Ammocoetes a long median tongue, which separates the most anterior branchial 
segments on each side. 

Finally, and this is the most conclusive evidence of all, this thyroid gland 
of Ammocoetes is totally unlike that of any of the higher vertebrates, and, 
indeed, of the adult form Petromyzon itself, but it forms an elaborate com- 
plicated organ, which is directly comparable with the uterus and genital ducts 
of animals such as scorpions. Not only is such a comparison valid with respect to 
its shape, but also with respect to its structure, which is absolutely unique among 
vertebrates, and very different to that of any other .vertebrate gland, but 
resembles in a striking manner a glandular structure found in the uterus, both 
of male and female scorpions. 

The generative glands in Limulus, together with the liver-glands, form a 
large glandular mass, situated in the head-region closely surrounding the central 
nervous system, so that the genital ducts pass from the head-region tailwards 
to the operculum. In the scorpion they lie in the abdominal region, so that 
their ducts pass head wards to the operculum. 

Probably in the Palieostraca the generative mass was situated in the cephalic 
region as in Limulus, and it is probable that the remnant of it still exists in 



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THE EVIDENCE OF THE THYROID GLAND 21 7 

Ammocoetes in the shape of the peculiar large cells packed together, with 
pigment masses in between them, which form such a characteristic feature of 
the glandular-looking material, which fills up the space between the cranial 
walls and the central nervous system. 

Finally, the relationship which has been known from time immemorial to 
exist between the sexual organs and the thyroid in man and other animals, and 
has hitherto been a mystery without any explanation, may possibly be the last 
reminiscence of a time when the thyroid glands were the uterine glands of the 
palieostracan ancestor. 

The consideration of the facial nerve, and the segments it supplies, still 
further points to the origin of the Vertebrata from the Palaaostraca. 



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CHAPTER VI 
THE EVIDENCE OF THE OLFACTORY APPARATUS 

Fishes divided into Amphirhince and Monorhinee. — Nasal tube of the lamprey. 
— Its termination at the infundibulum. — The olfactory organs of the 
scorpion group. — The camerostome. — Its formation as a tube. — Its deriva- 
tion from a pair of antennse. — Its termination at the true mouth. — Com- 
parison with the olfactory tube of Ammocoetes. — Origin of the nasal tube 
of Ammocoetes from the tube of the hypophysis.— Direct comparison of the 
hypophysial tube with the olfactory tube of the scorpion group — Summary. 

In the last chapter I finished the evidence given by the consideration 
of the mesosomatic or opisthotic nerves, and the segments they 
supplied. The evidence is strongly in accordance with that of 
previous chapters, and not only confirms the conclusion that verte- 
brates arose from some member of the PalsBOstraca, but helps still 
further to delimit the nature of that member. It is almost startling 
to see how the hypothesis put forward in the second chapter, sug- 
gested by the consideration of the nature of the vertebrate central 
nervous system and of the geological record, has received stronger 
and stronger confirmation from the consideration of the vertebrate 
optic apparatus, the vertebrate skeleton, the respiratory apparatus, 
and, finally, the thyroid gland. All fit naturally into a harmonious 
whole, and give a feeling of confidence that a similar harmony will 
be found upon consideration of the rest of the vertebrate organs. 

Following naturally upon the segments supplied by the opisthotic 
(mesosomatic) cranial nerves, we ought to consider now the segments 
supplied by the pro-otic (prosomatic) cranial nerves, i.e. the segments 
belonging to the trigeminal nerve-group in the vertebrate, and in the 
invertebrate the segments of the prosoma with their characteristic 
appendages. There are, however, in all vertebrates in this foremost 
cranial region, in addition to the optic nerves, two other well-marked 
nerves of special sense, tho olfactory and the auditory. Of these, 
the former are in the same class as the optic nerves, for they arise 



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THE EVIDENCE OF THE OLFACTORY APPARATUS 219 

in the vertebrate from the supra-infundibular nerve-mass, and in the 
invertebrate from the supra-cesophageal ganglia. The latter arise in 
the vertebrate from the infra-infundibular nerve-mass, and, as the 
name implies, are situated in the region where the pro-otic nerves 
are contiguous to the opisthotic, i.e. at the junction of the prosomatic 
and mesosomatic nerve-regions. 

The chapter dealing with the evidence given by the olfactory 
nerves and the olfactory apparatus ought logically to have followed 
immediately upon the one dealing with the optic apparatus, seeing 
that both these special sense-nerves belong to the supra-infundibular 
segments in the vertebrate and to the supra-cesophageal in the 
invertebrate. 

I did not deal with them in that logical sequence because it was 
necessary for their understanding to introduce first the conception of 
modified appendages as important factors in any consideration of 
vertebrate segments ; a conception which followed naturally after the 
evidence afforded by the skeleton in Chapter III., and by the branchial 
segments in Chapter IV. So, too, now, although the discussion of 
the prosomatic segmentation ought logically to follow immediately 
on that of the mesosomatic segmentation, I have determined to devote 
this chapter to the evidence of the olfactory organs, because the 
arguments as to the segments belonging to the trigeminal nerve- 
group are so much easier to understand if the position of the olfactory 
apparatus is first made clear. 



In all vertebrates the nose is double and opens into the pharynx, 
until we descend to the fishes, where the whole group Pisces has 
been divided into two subsidiary groups, Monorhime and Amphirhime, 
according as they possess a median unpaired olfactory opening, or a 
paired opening. The Mohorhime include only the Cyclostomata — the 
lampreys and hag-fishes. 

In the lampreys the single olfactory tube ends blindly, while in 
the hag-fishes it opens into the pharynx. In the lamprey, both in 
Petromyzon and Ammocoetes, the opening of this nasal tube is a 
conspicuous object on the dorsal surface of the head in front of the 
transparent spot which indicates the position of the right median 
eye. It is especially significant, as showing the primitive nature of 
this median olfactory passage, that a perfectly similar opening in the 



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220 THE ORIGIN OF VERTEBRATES 

same position is always found in the dorsal head-shields of all the 
Cephalaspid*e and Tremataspidae, as will be explained more fully in 
Chapter X. 

% All the evidence points to the conclusion that the olfactory 
apparatus of the vertebrate originated as a single median tube, con- 
taining the special olfactory sense-epithelium, which, although median 
and single, was innervated by the olfactory nerve of each side. The 
external opening of this tube in the lamprey is dorsal. How does it 
terminate ventrally ? 

The ventral termination of this tube is most instructive and 
suggestive. It terminates blindly at the very spot where the in- 
fundibular tube terminates blindly and the notochord ends. After 
transformation, when the Ammoccete becomes the Petromyzon, the 
tube still ends blindly, and does not open into the pharynx as in 
Myxine ; it, however, no longer terminates at the infundibulum, but 
extends beyond it towards the pharynx. 

This position of the nasal tube suggests that it may originally have 
opened into the tube of the central nervous system by way of the 
infundibular tube. This suggestion is greatly enhanced in value by 
the fact that in the larval Amphioxus the tube of the central nervous 
system is open to the exterior, its opening being known as the anterior 
neuropore, and this anterior neuropore is situated at the base of a pit, 
known as the olfactory pit because it is supposed to represent the 
olfactory organ of other fishes. 

Following the same lines of argument as in previous chapters, 
this suggestion indicates that the special olfactory organs of the 
invertebrate ancestor of the vertebrates consisted of a single median 
olfactory tube or passage, which led directly into the oesophagus and 
was innervated, though single and median, by a pair of olfactory 
nerves which arose from the supra-cesophageal ganglia. Let us see 
what is the nature of the olfactory organs among arthropods, and 
whether such a suggestion possesses any probability. 

The Olfactory Organs of the Scorpion Group. 

At first sight the answer appears to be distinctly adverse, for it is 
well known that in all the Insecta, Crustacea, and the large majority 
of Arthropoda, the first pair of antennae, often called the antennules, 
are olfactory in function, and these are free-moving, bilaterally 



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THE EVIDENCE OF THE OLFACTORY APPARATUS 221 

situated, independent appendages. Still, even here there is the strik- 
ing fact that the nerves of these olfactory organs always arise from 
the supra-cesophageal ganglia, although those to the second pair of 
antennie arise from the infra-cesophageal ganglia, just as the olfactory 
nerves of the vertebrate arise from the supra-infundibular brain-mass. 
Not only is there this similarity of position, but also a similarity of 
structure in the olfactive lobes of the brain itself of so striking a cha- 
racter as to cause Bellonci to sum up his investigations as follows : — 

" The structure and connections of the olfactive lobes present the 
same fundamental plan in the higher arthropods and in the verte- 
brates. In the one, as in the other, the olfactory fibres form, with 
the connecting fibres of the olfactory lobes, a fine meshwork, which, 
consisting as it does of separate groups, may each one be called an 
olfactory glomerulus." 

He attributes this remarkable resemblance to a physiological 
necessity that similarity of function necessitates similarity of structure, 
for he considers it out of the question to suppose any near relationship 
between arthropods and vertebrates. 

Truly an interesting remark, with the one fallacy that relationship 
is out of the question. 

The evidence so far has consistently pointed to some member of the 
palseostracan group as the ancestor of the vertebrates — a group which 
had affinities both to the crustaceans and the arachnids ; indeed, many 
of its members resembled scorpions much more than they resemble 
crustaceans. The olfactory organs of the scorpions and their allies are, 
therefore, more likely than any others to give a clue to the position of 
the desired olfactory organs. In these animals and their allies paired 
olfactory antennie are not present, either in the living land-forms or 
the extinct sea-scorpions, for all the antennae-like, frequently chelate, 
appendages seen in Pterygotus,etc. (Fig. 8), represent the chelicene,and 
correspond, therefore, to the second pair of antennas in the crustaceans. 

What, then, represents the olfactory antennas in the scorpions ? The 
answer to this question has been given by Croneberg, and very strik- 
ing it is. The two olfactory antennie of the crustacean have combined 
together to form a hollow tube at the base of which the mouth of the 
animal is situated, so that the food passes along this olfactory passage 
before it reaches the mouth. This organ is often called after Latreille, 
the camerostome, sometimes the rostrum ; it is naturally median in 
position and appears, therefore, to be an unpaired organ ; its paired 



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222 



THE ORIGIN OF VERTEBRATES 



pr.cnt 



character is, of course, evident enough, for it is innervated by a pair of 
nerves, and these nerves, as ought to be the case, arise from the supra- 
cesophageal ganglia. In Galeodes it is a conspicuously paired antennae- 
like organ (Fig. 94). 

Croneberg has also shown that this rostrum, or camerostome, arises 
embryologically as a pair of appendages similar to the other append- 
ages. This last observation 
of Croneberg has been con- 
firmed by Brauer in 1894, 
who describes the origin of 
the upper lip, as he calls it, 
in very similar terms, with- 
out, however, referring to 
Croneberg's paper. Crone- 
berg further shows that this 
foremost pair of antennae 
not only forms the so-called 
upper lip or camerostome, 
but also a lower lip, for 
from the basal part of the 
camerostome there projects 
on each side of the pharynx 
a dependent accessory por- 
tion, which in some cases 
fuses in the middle line, and 
forms, as it were, a lower lip. 
The entosclerite belonging 
to this dependent portion is 
apparently the post - oral 
entosclerite of Lankester and 
Miss Beck. 

At the base of the tubular 
passage formed by this modified first pair of antennae the true mouth 
is found opening directly into the dilated pharynx, the muscles 
of which enable the act of suction to be carried out. The narrow 
oesophagus leads out from the pharynx and is completely surrounded 
by the supra- and infra-oesophageal nerve masses. 

Huxley also describes the moutli of the scorpion in precisely the 
same position (cf. o, Fig. 96). 




Fig. 94. — Dorsal View of Brain and Came- 
b08tome of galeodes. 

aim., camerostome ; pr. ent. t p re-oral entoscle- 
rite ; l.L, depeudeut portion of camerostome ; 
ph., pharynx; al., alimentary canal; n. op., 
median optic nerves; pi., plastron; v.c. y 
ventral nerve chain ; 2, 3, second and third 
appendages. 



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THE EVIDENCE OF THE OLFACTORY APPARATUS 223 

In order to convey to my readers the antennae-like character of the 
camero8tome in Galeodes (Fig. 101), and its position, I give a figure 
(Fig. 94) of the organ from its dorsal aspect, after removal of the 
chelicerae and their muscles. A side view of the same organ is given 
in Fig. 95 to show the feathered termination of the camerostome, 
and the position of the dependent accessory portion (LI.) (Crone- 
berg's ' untere Anhang ') with its single long antenna-like feather. 
In both figures the alimentary canal (al.) is seen issuing from the 
conjoined supra- and infra-cesophageal mass. 

As is seen in the figures, the bilateral character of the rostrum, as 
Croneberg calls it, is apparent not only in its feathered extremity 
but also in its chitinous covering, the softer median dorsal part (left 

p'cnt 




Fig. 95.— Lateral View op Brain and Camerostome op Galeodes. 

gl. supr. as., supra-cesophageal ganglion; gl. infr. ces., infra-oesophageal ganglion. 
The rest of the lettering same as in Fig. 94. 

white in figure) being bounded by two lateral plates of hard chitin, 
which meet in the middle line near the extremity of the organ. In 
all the members of the scorpion group, as is clearly shown in Crone- 
berg's figures, the rostrum or camerostome is built up on the same 
plan as in Galeodes, though the antenna-like character may not be 
so evident. 

When we consider that the first pair of antennae in the crustaceans 
are olfactory in function, Croneberg s observations amount to this — 

In the arachnids and their allies the first pair of antennae form 
a pre-oral passage or tube, olfactory in function ; the small mouth, 
which opens directly into the pharynx, being situated at the end 
of this olfactory passage. 



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224 



THE ORIGIN OF VERTEBRATES 



Croneberg's observations and conclusions are distinctly of very 
great importance in bringing the arachnids into line with the crus- 
taceans, and it is therefore most surprising that they are absolutely 
ignored by Lankester and Miss Beck in their paper published in 
1883, in which Latreille only is mentioned with respect to this 
organ, and his term " camerostome," or upper lip, is used throughout, 
in accordance with the terminology in Lankester's previous paper. 
That this organ is not only a movable lip or tongue, but essentially 
a sense-organ, almost certainly of smell and taste, as follows from 
Croneberg's conclusions, is shown by the series of sections which I 
have made through a number of young Thelyphonus (Fig. 102). 



pr ent 




01 f pa$B 



Fig. 96.— Median Sagittal Section thbough a Young Thelyphonus. 

I give in Fig. 96 a sagittal median section through the head-end 
of the animal, which shows clearly the nature of Croneberg's con- 
ception. At the front end of the body is seen the median eye (cc), 
o is the mouth, Ph. the pharynx, m. the narrow oesophagus, com- 
pressed between the supra-cesophageal (supr. as.) and infra-cesopha- 
geal (in/r. ces.) brain mass, which opens into the large alimentary 
canal (Al.) ; Olf. pass, is the olfactory passage to the mouth, lined 
with thick- set, very fine hairs, which spring from the hypostome 
(Hyp.) as well as from the large conspicuous camerostome (Cam.) y 
which limits this tube anteriorly. The space between the came- 
rostome and the median eye is filled up by the massive chelicerae, 
which are not shown in this section, as they begin to appear in the 



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THE EVIDENCE OF THE OLFACTORY APPARATUS 225 

sections on each side of the median one. The muscles of the pharynx 
and the muscles of the camerostome are attached to the pre-oral 
entosclerite {pr. ent.). The post-oral entosclerite is shown in section 
as post. ent. The dorsal blood-vessel, or heart, is indicated at H. 

In Fig. 97 I give a transverse section through another specimen 
of the same litter, to show the nature of this olfactory tube when cut 
across. Both sections show most clearly that we are dealing here with 
an elaborate sense-organ, the surface of which is partly covered with 
very fine long hairs, partly, as is seen in the figure, is composed of 
long, separate, closely-set sense-rods (bat.), well protected by the 
long hairs which project on every side in front of them, which recall 
to mind Bellonci's figure of the ' batonnets olfactives ' on the antennae 
of Sphaeroma. Finally, we have the observation of Blanchard quoted 
by Huxley, to the effect that this camerostome is innervated by 
nerves from the supra-cesophageal ganglia which are clearly bilateral, 
seeing that they arise from the ganglion on each, side and then unite 
to pass into the camerostome ; in other words, paired olfactory nerves 
from the supra-oesophageal ganglia. 

These facts demonstrate with wonderful clearness that in one 
group of the Arthropoda the olfactory antennre have been so modi- 
fied as to form an olfactory tube or passage, which leads directly 
into the mouth and so to the oesophagus of the animal, and, strikingly 
enough, this group, the Arachnida, is the very one to whicli the 
scorpions belong. 

If for any cause the mouth in Fig. 96 were to be closed, then 
the olfactory tube (olf. pass.) might still remain, owing to its impor- 
tance as the organ of smell, and the olfactory tube would terminate 
blindly at the very spot where the corresponding tube does terminate 
in the vertebrate, according to the theory put forward in this book. 

The Olfactory Tube of Ammoccetes. 

In all cases where there is similarity of topographical position 
in the organs of the vertebrate and arthropod we may expect also to 
find similarity of structure. At first sight it would appear as though 
such similarity fails us here, for a cross-section of the olfactory tube 
in Petromyzon represents an elaborate organ such as is shown in Fig. 
98, very different in appearance to the section across the olfactory 
passage of a young Thelyphonus given in Fig. 97. 

Q 



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226 



THE ORIGIN OF VERTEBRATES 







Fig. 97. — Transverse Section through the Olfactory Passage op a Young 

Thelyphonus. 

1 and 2, sections of first and second appendages. 




-cart. 



Fig. 98.— Transverse Section through the Olfactory Passage of Petromyzon. 

cart., nasal cartilage. 



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THE EVIDENCE OF THE OLFACTORY APPARATUS 227 

As is seen,, it is difficult to see any connection between these 
folds of olfactory epithelium and the simple tube of the scorpion. 
But in the nose, as in all other parts of the head-region of the 
lamprey, remarkable changes take place at transformation, and 
examination of the same tube in Ammocoetes demonstrates that the 
elaborate structure of the adult olfactory organ is actually derived 
from a much simpler form of organ, represented in Fig. 99. Here, 
in Ammoccetes, the section is no longer strikingly different from that 
of the Thelyphonus organ, but, instead, most strikingly similar to it. 
Thus, again, it is shown that this larval form of the lamprey gives 




-cart 



Fig. 99.— Transverse Section through the Olfactoby Passage op Ammoccetes. 

cart., nasal cartilage. 

more valuable information as to vertebrate ancestry than all the 
rest of the vertebrates put together. 

Still, even now the similarity between the two organs is not 
complete, for the tube in the lamprey opens on to the exterior on the 
dorsal surface of the head, while in the scorpion tribe it is situated 
ventrally, being the passage to the mouth and alimentary canal. In 
accordance with this there is no sign of any opening on the dorsal 
carapace of any of the extinct sea-scorpions or of the living land- 
scorpions, such as is so universally found in the cephalaspids, trema- 
taspids, and lampreys. Here is a discrepancy of an apparently 
serious character, yet so wonderfully does the development of the 
individual recapitulate the development of the race, that this very 
discrepancy becomes converted into a triumphant vindication of the 



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2 28 THE ORIGIN OF VERTEBRATES 

correctness of the theory advocated in this book, as soon as we turn 
our attention to the development of this nasal tube in the lamprey. 

We must always remember not only the great importance of a lar- 
val stage for the unriddling of problems of ancestry, but also the great 
advantage of being able to follow more favourably any clues as to 
past history afforded by the development of the larva itself, owing 
to the greater slowness in the development of the larva than of the 
embryo. Such a clue is especially well marked in the course of 
development of Ammoccetes according to Kupffer's researches, for 
he finds that when the young Ammoccetes is from 5 to 7 mm. in 
length, some time after it has left the egg, when it is living a free 
larval life, a remarkable series of changes takes place with consider- 
able rapidity, so that we may regard the transformation which takes 
place at tliis stage, as in some degree comparable with the great trans- 
formation which occurs when the Ammoccetes becomes a Petromyzon. 

All the evidence emphasizes the fact that the latter transformation 
indicates the passage from a lower into a higher form of vertebrate, 
and is to be interpreted phylogenetically as an indication of the 
passage from the Cephalaspidian towards the Dipnoan style of fish. 
If, then, the former transformation is of the same character, it would 
indicate the passage from the Paheostracan to the Cephalaspid. 

What is the nature of this transformation process as described 
by Kupffer ? 

It is characterized by two most important events. In the first 
place, up to this time the oral chamber has been cut off from the 
respiratory chamber by a septum — the velum — so that no food could 
pass from the mouth to the alimentary canal. At this stage the 
septum is broken through, the oral chamber communicates with the 
respiratory chamber, and the velar folds of the more adult Ammoccetes 
are left as the remains of the original septum. The other striking 
change is the growth of the upper lip, by which the orifice of the nasal 
tube is transferred from a ventral to a dorsal position. Fig. 100, 
taken from Kupffer's paper, represents a sagittal section through an 
Ammocates 4 mm. long; /./. is the lower lip, u.l. the upper lip, and, 
as is seen, the short oral chamber is closed by the septum, vel. Open- 
ing ventially is a tube called the tube of the hypophysis, Hy. t which 
extends close up to the termination of the infundibulum. On the 
anterior surface of this tube is the projection called by Kupffer the 
olfactory plakode. At this stage the upper lip grows with great 



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THE EVIDENCE OF THE OLFACTORY APPARATUS 229 

rapidity and thickens considerably, thus forcing the opening of the 
hypophysial tube more and more dorsalwards, until at last, in the full- 
grown Ammoccetes, it becomes the dorsal opening of the nasal tube, 
as already described. Here, then, in the hypophysial tube we have 
the original position of the olfactory tube of the vertebrate ancestor, 
and it is significant, as showing the importance of this organ, to find 
that such a hypophysial tube is characteristic of the embryological 
development of every vertebrate, whatever may be the ultimate form 
of the external nasal orifices. 

The single median position of the olfactory organ in the Cyclo- 
stomata, in contradistinction to its paired character in the rest of the 




Hij u iOr 11 vel 
Fig. 100.— Ganglia op the Cranial Nerves op an Ammoc(etks, 4 mm. in length, 

PROJECTED ON TO THE MEDIAN PLANE. (After KUPFFKR.) 

A-B, the line of c pi branchial ganglia; au., auditory capsule; nc., notochord; Hy., 
tube of hypophysis ; Or., oral cavity ; u .1., upper lip ; l.l. lower lip ; vel., septum 
between oral and respiratory cavities; V., VII., IX., X., cranial nerves; a*., 
nerve with four epibranchial ganglia. 

vertebrates, has always been a stumbling-block in the way of those 
who desired to consider the Cyclostomata as degenerated Selachians, 
for the origin of the olfactory protuberance, as a single median 
plakode, seemed to indicate that the nose arose as a single organ and 
not as a paired organ. 

On the other hand, the two olfactory nerves of Ammocrutes 
compare absolutely with the olfactory nerves of other vertebrates, 
and force one to the conclusion that this median organ of Ammo- 
cu'tes arose from a pair of bilateral organs, which have fused in the 
middle line. 

The comparison of this olfactory organ with the camerostome 



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23O THE ORIGIN OF VERTEBRATES 




Pig, L01.— Gakode*. (From the Koval Natural Hid 






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THE EVIDENCE OF THE OLFACTORY APPARATUS 23 1 

gives a satisfactory reason for its appearance in the lowest verte- 
brates as an unpaired median organ ; equally so, the history of the 
camerostome itself supplies the reason why the olfactory nerves are 
double, why the organ is in reality a paired organ and not a single 




Fig. 102.— Thelyphonus. (From the Royal Natural History.) 

median one. Thus, in a sense, the grouping of the fishes into Mono- 
rhinae and Amphirhime has not much meaning, seeing that the 
olfactory organ is in all cases double. 

The evidence of the olfactory organs in the vertebrate not only 
confirms, in a most striking manner, the theory of the origin of the 



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232 THE ORIGIN OF VERTEBRATES 

vertebrate from the Palaeostracan, but points indubitably to an origin 
from a scorpion-like rather than a crustacean-like stock. To com- 
plete the evidence, it ought to be shown that the ancient sea-scorpions 
did possess an olfactory passage similar to the modern land-scorpions. 
The evidence on this question will come best in the next chapter, 
where I propose to deal with the prosomatic appendages of the Palse- 
ostracan group. 

Summary. 

The vertebrate olfactory apparatus commences as a single median tube 
which terminates dorsally in the lamprey, and is supplied by the two olfactory 
nerves which arise from the supra-infundibular portion of the brain. It is 
a long, tapering" tube which passes ventrally and terminates blindly at the 
infundibulum in Ammocoetes. The dorsal position of the nasal opening is not 
the original one, but is brought about by the growth of the upper lip. The 
nasal tube originally opened ventrally, and was at that period of development 
known as the tube of the hypophysis. 

The evidence of Ammocoetes thus goes to show that the olfactory 
apparatus started as an olfactory tube on the ventral side of the animal, which 
led directly up to, and probably into, the oesophagus of the original alimentary 
canal of the palaeostracan ancestor. 

Strikingly enough, although in the crustaceans the first pair of antennw 
form the olfactory organs, no such free antennae are found in the araclinids, 
but they have amalgamated to form a tube or olfactory passage, which leads 
directly into the mouth and oesophagus of the animal. 

This olfactory passage is very conspicuous in all members of the scorpion 
group, and, like the olfactory tube of the vertebrate, is innervated by a pair of 
nerves, which resemble those supplying the first pair of antennae in crustaceans 
as to their origin from the supra-oesophageal ganglia. 

This nasal passage, or tube of the hypophysis, corresponds in structure and 
in position most closely with the olfactory tube of the scorpion group, the only 
difference being that in the latter case it opens directly into the oesophagus, 
while in the former, owing to the closure of the old mouth, it cannot open into 
the infundibulum. 

The evidence of the olfactory apparatus, combined with that of the optic 
apparatus, is most interesting, for, whereas the former points indubitably to an 
ancestor having scorpion-like affinities, the structure of the lateral eyes points 
distinctly to crustacean, as well as arachnid, affinities. 

Taking the two together the evidence is extraordinarily strong that the 
vertebrate arose from a member of the palaaostracan group with marked 
scorpion-like affinities. 



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CHAPTER VII 
THE PROSOMA TIC SEGMENTS OF LIMULUS AND ITS ALLIES 

Comparison of the trigeminal with the prosomatic region. — The prosomatic 
appendages of the Gigantostraca. — Their number and nature. — Endognaths 
and ectognath. — The metastoma. — The coxal glands. — Prosomatic region 
of Eurypterus compared with that of Ammocoetes. — Prosomatic segmenta- 
tion shown by muscular markings on carapace. — Evidence of coelomic 
cavities in Limulus. — Summary. 

The derivation of the olfactory organs of the vertebrate from the 
olfactory antennae of the arthropod in the last chapter is confirmatory 
proof of the soundness of the proposition put forward in Chapter IV., 
that the segmentation in the cranial region of the vertebrate was 
derived from that of the prosomatic and mesosomatic regions of the 
palaeostracan ancestor. Such a segmentation implies a definite series 
of body-segments, corresponding to the mesomeric segmentation of 
the vertebrate, and a definite series of appendages corresponding to 
the splanchnic segmentation of the vertebrate. 

Of the foremost segments belonging to the supra-( esophageal 
region characterized by the presence of the median eyes, of the lateral 
eyes, and of the olfactory organs, a wonderfully exact replica has 
been shown to exist in the pineal eyes, the lateral eyes, and the 
olfactory organ of the vertebrate, belonging, as they all do, to the 
supra-infundibular region. 

Of the infra- esophageal segments belonging to the prosoma and 
mesosoma respectively, the correspondence between the mesosomatic 
segments carrying the branchial appendages and the uterus, witli 
those in the vertebrate carrying the branchiae and the thyroid gland 
respectively, has been fully proved in previous chapters. 

There remain, then, only the segments of the prosomatic region 
to be considered, a region which, both in the vertebrate and inver- 
tebrate, is never respiratory in function but always masticatory, such 



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234 THE ORIGIN OF VERTEBRATES 

mastication being performed in Limulus and its allies by the muscles 
which move the foot-jaws or gnathites, which are portions of the 
prosomatic appendages specially modified for that purpose, and in the 
vertebrates by the masticatory muscles, which are always innervated 
by the trigeminal or Vth cranial nerve. This comparison implies 
that the motor part of the trigeminal nerve originally supplied the 
prosomatic appendages. 

The investigations of van Wijhe and of all observers since the 
publication of his paper prove that in this trigeminal region, as in the 
vagus region, a double segmentation exists, of which the ventral or 
splanchnic segments, corresponding to the appendages in the inver- 
tebrate, are supplied by the trigeminal nerves, while the dorsal or 
somatic segments, corresponding to the somatic segments in the 
invertebrate, are supplied by the Illrd or oculomotor and the I Vth 
or trochlear nerves — nerves which supply muscles moving the lateral 
eyes. 

In accordance, then, with the evidence afforded by the nerves of 
the branchial segments, it follows that the muscles supplied by the 
motor part of the trigeminal ought originally to have moved the ap- 
pendages belonging to a series of prosomatic segments. On the other 
hand, the eye-muscles ought to have belonged to the body-part of the 
prosomatic segments, and must therefore have been grouped origi- 
nally in a segmental series corresponding to the prosomatic appendages. 

The evidence for and against this conclusion will be the subject 
of consideration in this and the succeeding chapters. At the outset 
it is evident that any such comparison necessitates an accurate know- 
ledge of the number of the prosomatic segments in the Gigantostraca 
and of the nature of the corresponding appendages. 

In all this group of animals, the evidence as to the number of 
segments in either the prosomatic or mesosomatic regions is given 

by- 

1. The number of appendages. 

2. The segmental arrangement of the muscles of the prosoma or 
mesosoma respectively. 

3. The segmental arrangement of the ccelomic or head-cavities. 

4. The divisions of the central nervous system, or neuromeres, 
together with their outgoing segmental nerves. 

It follows, therefore, that if from any cause the appendages are 
not apparent, as is the case in many fossil remains, or have dwindled 



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PROSOMATIC SEGMENTS OF LIMULUS 235 

away and become insignificant, we still have the muscular, ccelomic, 
and nervous arrangements left to us as evidence of segmentation in 
these animals, just as in vertebrates. 

In this prosomatic region, we find in Limulus the same tripartite 
division of the nerves as in the mesosomatic region, so that the 
nerves to each segment may be classed as (1) appendage-nerve; 
(2) sensory or dorsal somatic nerve, supplying the prosomatic cara- 
pace ; (3) motor or ventral somatic nerve, supplying the muscles of 
the prosoma, and containing possibly some sensory fibres. The main 
difference between these two regions in Limulus consists in the closer 
aggregation of the prosomatic nerves, corresponding to the concentra- 
tion of the separate ganglia of origin in the prosomatic region of the 
brain. 

The number of prosomatic segments in Limulus is not evident 
by examination of the prosomatic carapace, so that the most reliable 
guide to the segmentation of this region is given by the appendages, 
of which one pair corresponds to each prosomatic segment. 

The number of such segments, according to present opinion, is 
seven, viz. : — 

(1) The foremost segment, which bears the chelicerae. 

(2, 3, 4, 5, 6) The next five segments, which carry the paired 
locomotor appendages ; and 

(7) The last segment, to which belongs a small abortive pair of 
appendages, known by the name of the chilaria, situated between the 
last pair of locomotor appendages and the operculum or first pair of 
mesosomatic appendages. These appendages are numbered from 1-7 
in the accompanying drawing (Fig. 103). 

Of these seven pairs of appendages, the significance of the first 
and the last has been matter of dispute. With respect to the first 
pair, or the chelicer®, the question has arisen whether their nerves 
belong to the infra-oesophageal group, or are in reality supra- 
cesophageal. 

It is instructive to observe the nature and the anterior position of 
this pair of appendages in the allied sea-scorpions, especially in Ptery- 
gotus, where the only chelate organs are found in these long, antennae- 
like chelicerae. In Slimonia and in Stylonurus they are supposed by 
Woodward to be represented by the small non -chelate antennae seen 
in Fig. 8, B and C (p. 27), taken from Woodward. If such is the case, 
then these figures show that a pair of appendages is missing in each 



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236 



THE ORIGIN OF VERTEBRATES 



of these forms, for they possess only five free prosomatic appendages 
instead of six, as in Limulus and in Pterygotus. Similarly, Wood- 
ward only allowed five appendages for Pterygotus, so that his restora- 
tions were throughout consistent. Schmidt, in Pterygotus osiliensis 
has shown that the true number was six, not five, as seen in his 
restoration given in Fig. 8, A (p. 27). 

With respect to Eurypterus, Schmidt figures an exceedingly 




Fig. 103.— Ventral Surface op Limulus. (Taken from Kishinouve.) 

The gnathic bases of the appendages have been separated from thoso of the other 
side to show the promesosternite or endostoma (End.). 

minute pair of antennae between the coxal joints of the first pair of 
appendages,, thus making six pairs of appendages. Gerhard Holm, 
however, in his recent beautiful preparations from Schmidt's specimens 
and others collected at Kootzikull, has proved most conclusively that 
the chelicerae of Eurypterus were of the same kind as those of 
Limulus. I reproduce his figure (Fig. 104) showing the small chelate 
chelicene (1) overhanging the mouth orifice, just as in Limulus or in 
Scorpio. 



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PROSOMATIC SEGMENTS OF LIMULUS 



m 



So, also, since Woodward's monograph, Laurie has discovered in 
Slimonia acuminata a small median pair of chelate appendages 
exactly corresponding to the chelicene of Limulus, or of Eurypterus, 
or of Scorpio. We may, therefore, take it for granted that such was 
also the case in Stylonurus, and that the foremost pair of proso- 
matic appendages in all these extinct sea-scorpions were in the same 
position and of the same character as the chelicera? of the scorpions. 

In the living scorpion and in Limulus the nerves to this pair of 




Fig. 104.— Eurypterus Fischeri. (From Holm.) 

appendages undouhtedly arise from the foremost prosomatic ganglia, 
and the reason why they appear to belong to the supra- oesophageal 
brain-mass has been made clear by Brauer's investigations on the 
embryology of Scorpio ; for he has shown that the cheliceral ganglia 
shift from the ventral to the dorsal side of the cesophagus during 
development, thus becoming pseudo-supra-cesophageal, though in 
reality belonging to the infra-a*sophageal ganglia. This cheliceral 
pair of appendages is, in all probability, homologous with the second 
pair of antemue in the Crustacea. 



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238 THE ORIGIN OF VERTEBRATES 

I conclude, then, that the chelicerae must truly be included in 
the prosomatic group, but that they stand in a somewhat different 
category to the rest of the prosomatic appendages, inasmuch as they 
take up a very median anterior and somewhat dorsal position, and 
their ganglia of origin are also exceptional in position. 

Next for consideration come the chilaria (7 in Fig. 103), which 
Lankester did not consider to belong to appendages at all, but to 
be a peculiar pair of sternites. Yet their very appearance, with 
their spinous hairs corresponding to those of the other gnathites and 
their separate nerve-supply, all point distinctly to their being a 
modified pair of appendages, and, indeed, the matter has been placed 
beyond doubt by the observations of Kishinouye, who has found 
embryologically that they arise in the same way as the rest of the 
prosomatic appendages, and belong to a distinct prosomatic segment, 
viz. the seventh segment. In accordance with this, Brauer has found 
that in the scorpion there is in the embryo a segment, whose ap- 
pendages degenerate, which is situated between the segment bearing 
the last pair of thoracic appendages and the genital operculum — a 
segment, therefore, comparable in position to the chilarial segment of 
Limulus. 

Coming now to the five locomotor appendages, we find that they 
resemble each other to a considerable extent in most cases, with, 
however, certain striking differences. Thus in Limulus they are 
chelate, with their basal joints formed as gnathites, except in the 
case of the fifth appendage, in which the extremity is modified for 
the purpose of digging in the sand. In Pterygotus, Slimonia, Euryp- 
terus, the first four of these appendages are very similar, and are 
called by Huxley and Woodward endognaths; in all cases they 
possess a basal part or sterno-coxal process, which acts as a gnathite 
or foot-jaw, and a non-chelate tactile part, which possesses no pre- 
hensile power, and in most cases could have had no appreciable 
share in locomotion, called by Huxley and Woodward the palpus. 
These small palps were probably retractile, and capable of being 
withdrawn entirely under the hood. The fifth appendage is usually 
different, being a large swimming organ in Pterygotus, Eurypterus, 
and Slimonia (Figs. 8 and 104), and is known as the ectognath. 

Finally, in Drepanopterus Bembycoidts, as stated by Laurie, all 
five locomotor appendages are built up after the same fashion, the 
last one not being formed as a paddle-shaped organ or elongated as 



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PROSOMATIC SEGMENTS OF LIMULUS 239 

in Stylonurus, but all five possess no special locomotor or prehensile 
power. According to Laurie this is a specially primitive form of the 
group. 

It is significant to notice from this sketch that with the absence 
of special prehensile terminations such as chelae, or the absence of 
special locomotor functions such as walking or swimming, these 
appendages tend to dwindle and become insignificant, taking up the 
position of mere feelers round the mouth, and at the same time are 
concentrated and pressed closely together, so that their appendage- 
nerves must also be close together. 

This sketch therefore shows us that — 

Of the six foremost prosomatic appendages, the chelicera and the 
four endognaths were, at the time when the vertebrates first appeared, 
in very many cases dwindling away ; the latter especially no longer 
functioned as locomotor appendages, but were becomiug more and 
more mere palps or tentacles situated round the mouth, which could 
by no possibility afford any help to locomotion. 

On the contrary, the sixth pair of appendages — the ectognaths — 
remained powerful, being modified in many cases into large oar-like 
limbs by which the animal propelled itself through the water. 

It is a striking coincidence that those ancient fishes, Ptericthys 
and Bothriolepis, should have possessed a pair of large oar-like 
appendages. 

At this time, then, in strong contrast to the endognaths, the 
ectognaths, or sixth pair of appendages, remained strong and vigo- 
rous. What about the seventh pair, the chilaria of Limulus ? 

Of all the prosomatic appendages these are the most interesting 
from the point of view of my theory, for whereas in the scorpion of 
the present day they have dwindled away and left no trace except in 
the embryo, in the sea-scorpions of old, far froin dwindling, they 
had developed and become a much more important organ than the 
chilaria of Limulus. 

In all these animals a peculiarly striking and unique structure is 
found in this region known by the name of the metastoma, or lip-plate 
(Figs. 8 and 104 (7)) ; it is universally considered to be formed by 
the fusion of the two chilarial appendages. 

All observers are agreed that this lip-plate was freely movable. 
Nieskowski considers that the movement of the metastoma was 
entirely in a vertical direction, whereby the cleft which is seen 



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240 



THE ORIGIN OF VERTEBRATES 



between the basal joints of all the pairs of locomotor appendages 
could be closed from behind. Woodward says it no doubt represents 
the labium, and served more effectually to enclose the posterior part 
of the buccal orifice, being found exteriorly to the toothed edges of the 
ectognaths or maxillipedes. Schmidt agrees with Nieskowski, and 




nobi 





01f > Hyp I En* 

COX.5I 5 

Fig. 105. — Diagram of Sagittal Median Sfxtion through A, Limulus, B, 

EURYFTERUS. 

looks on the mestasoma as forming a lower lip within which the 
bases of the ectognaths worked. 

Quite recently Gerhard Holm has worked over again the very 
numerous specimens of Eurypterus Fisclicri, which are obtainable at 
Rootzikiill, and has thrown new light on the relation of the metas- 
toma to the mouth-parts. His preparations show clearly that the 
true lower lip of Eurypterus was not the metastoma, for when the 
metastoma is removed another plate {End., Fig. 105, B) situated 



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PROSOMATIC SEGMENTS OF LIMULUS 2\\ 

internally to it is disclosed, which, in his view, corresponds to the 
sternite between the bases of the pro-somatic appendages in Limu- 
lus, i.e. to the sternite called by Lankester, the pro-mesosternite (End., 
Fig. 103). This inner plate formed with the metastoma ((7) Fig. 
105) and the ectognaths (6) a chamber closed posteriorly, within 
which the bases of the ectognaths worked. In other words, the 
removal of the metastoma discloses in Eurypterus the true anterior 
ventral surface of the animal which corresponds to that of Limulus, 
or of the scorpion group, with its pro-mesosternite and laterally 
attached gnathites or sterno-coxal processes. To this inner plate or 
pro-mesosternite Holm gives the name of endostoma. 

To the anterior edge of the endostoma a thinner membrane is 
attached which passes inwards in the direction of the throat, and 
forms, therefore, the lower lip (Hyp., Fig. 105, B) of the passage of 
the mouth (olf.p.). This membrane bears upon its surface a tuft of 
hairs, which he thought were probably olfactory in function. Con- 
sequently, in his preliminary communication, he describes this lower 
lip as forming, in all probability, an olfactory organ ; in his full 
communication he repudiates this suggestion, because he thinks it 
unlikely that such an organ would be situated within the mouth. I 
feel sure that if Holm had referred to Croneberg's paper, and seen 
how the true mouth in all the scorpion group is situated at the base 
of an olfactory passage, he would have recognized that his first sug- 
gestion is in striking accordance with the nature of the entrance to 
the mouth in other scorpions. 

That Eurypterus also possessed a camerostome (ram.) seems to 
follow of necessity from its evident affinities both with Limulus 
and the scorpions. We see, in fact, that the mouth of these old sea- 
scorpions was formed after the fashion of Limulus, surrounded by 
masticatory organs in the shape of foot-jaws, and yet foreshadowed 
that of the scorpion, so that an ideal sagittal section of one of these 
old palu»ostracan forms would be obtained by the combination of 
actual sagittal sections through Limulus and a member of the scorpion 
group, with, at the same time, a due recognition of Holm's researches. 
Such a section is represented in Fig. 105, B, in whicli I have drawn 
the central nervous system and its nerves, the median eyes (C.E.), 
the olfactory organs (Cam.), the pharynx (Ph.), u'sophagus (<«*.), and 
alimentary canal (Al.), but have not tried to indicate the lateral eyes. 
I have represented the prosomatic appendages by numbers (1-7), and 



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242 THE ORIGIN OF VERTEBRATES 

the foremost mesosomatic segments by numbers (8-13). I have 
placed the four endognaths and the nerves going to them close 
together, and made them small, mere tentacles, in recognition of the 
character of these appendages in Eurypterus, and have indicated the 
position and size of the large ectognath, with its separate nerve, 
by (6). If among the ancient Eurypterus-like forms, which were 
living at the time when vertebrates first appeared, there were some 
in which the ectognaths also had dwindled to a pair of tentacles, 
then such animals would possess a prosomatic chamber formed by 
a metastoma or accessory lip, within which were situated five pairs 
of short tactile appendages or tentacles. If the vertebrate were 
derived from such an animal, then the trigeminal nerve, as the 
representative of these prosomatic appendage-nerves, ought to be 
found to supply the muscles of this accessory lip and of these five 
pairs of tentacles in the lowest vertebrate. 

This prosomatic or oral chamber, as it might be called, was limited 
posteriorly by the fused metastoma (7) and operculum (8), so that if 
in the same imaginary animal one imagines that the gill-chambers, 
instead of being separate, are united to form one large respiratory 
chamber, then, in such an animal, a prosomatic oral chamber, in 
which the prosomatic appendages worked, would be separated from 
a mesosomatic respiratory chamber by a septum composed of the 
conjoined basal portions of the mesosomatic operculum and the 
prosomatic metastoma, as indicated in the diagram. In this septum 
the nerves to the last prosomatic appendage (equivalent to the last 
part of the trigeminal in the vertebrate) and to the first mesosomatic 
(equivalent to the thyroid part of the facial) would run, as shown in 
the figure, close together in the first part of their course, and would 
separate when the ventral surface was reached, to pass headwards 
and tailwards respectively. 

The Coxal Glands. 

One more characteristic of these appendages requires mention, 
and that is the excretory glands situated at the base of the four 
endognaths known as the coxal glands. These glands are the main 
excretory organs in Limulus and the scorpions, and extend into the 
basal segments or coxae of the four endognaths, not into those of the 
ectognaths or the chilaria (or metastoma). Hence their name, coxal 



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PROSOMATIC SEGMENTS OF LIMULUS 243 

glands ; and, seeing the importance of the excretory function, it is 
likely enough that they would remain, even when the appendages 
themselves had dwindled away. With the concentration and 
dwindling of the endognaths these coxal glands would also be con- 
centrated, so that in the diagram (Fig. 105) they would rightly be 
grouped together in the position indicated (cox. gl.). 

Such a diagram indicates the position of all the important organs 
of the head-region except the special organs for taste and hearing. 
These, for the sake of convenience, I propose to take separately, in 
order at this stage of my argument not to overburden the simplicity 
of the comparison I desire to make with too much unavoidable detail. 

TflE PltOSOMATIC EEGION OF AMMOCCETES. 

Let us now compare this diagram with that of the corresponding 
region in Ammoccetes and see whether or no any points of similarity 
exist. 

With respect to this region, as in so many other instances already 
mentioned, Ammoccetes occupies an almost unique position among 
vertebrates, for the region supplied by the trigeminal nerve— the 
prosomatic region — consists of a large oral chamber which was 
separated from the respiratory chamber in the very young stage by 
a septum which is subsequently broken through, and so the two 
chambers communicate. 

This chamber is bounded by the lower lip ventrally, the upper 
lip and trabecular region dorsally, and the remains of the septum or 
velum laterally and posteriorly. It contains a number of tentacles 
arranged in pairs within the chamber so as to form a sieve-like fringe 
inside the circular mouth ; of these, the ventral pair are large, fused 
together, and attached to the lower lip. 

All the muscles belonging to this oral chamber are of the 
visceral type, and are innervated by the trigeminal nerve. In 
accordance with the evidence obtained up to this point this means 
that such an oral chamber was formed by the prosomatic appendages 
of the invertebrate ancestor, similarly to the oral chamber just figured 
for Eurypterus. 

This chamber in the full-grown Ammoccetes is not only open to 
the respiratory chamber, but is bounded by the large upper lip (U.L., 
Fig. 106, D). On the dorsal surface of this region, in front of the 



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244 THE ORIGIN OF VERTEBRATES 

pineal eye (C.E.), is the most conspicuous opening of the olfactory 
tube (Na.) f which olfactory tube passes from the dorsal region to the 
ventral side to terminate blindly at the very spot where the infun- 
dibulum comes to the surface of the brain. Here, also, is situated 
that extraordinary glandular organ known as the pituitary body 
(Pit.). A sagittal section, then, in diagram form, of the position 
of parts in the full-grown Ammocu'tes, would be represented as in 
Fig. 106, D. 

But, as argued out in the last chapter, the diagram of the adult 
Ammoccetes must be compared with that of a cephalaspidian fish ; 
the diagram of the palseostracan must be compared with the larval 
condition of Ammoccetes. In other words, Fig. 106, B, must be 
compared with Fig. 106, C, which represents a section through the 
larval Ammoccetes as it would appear if it reached the adult con- 
dition without any forward growth of the upper lip or any breaking 
through of the septum between the oral and respiratory chambers. 
The striking similarity between this diagram and that of Euryp- 
terus becomes immediately manifest even to the smallest details. 
The only difference between the two, except, of course, the notochord, 
consists in the closure of the mouth opening (o), in Fig. 106, B, by 
which the olfactory passage (olf. p.) of the scorpion becomes con- 
verted into the hypophysial tube (Hy), Fig. 106, C, and later into 
the nasal tube (Na.), Fig. 106, D, of the full-grown Ammoccetes. 
That single closure of the old mouth is absolutely all that is 
required to convert the Eurypterus diagram into the Ammoccetes 
diagram. 

Such a comparison immediately explains in the simplest manner 
a number of anatomical peculiarities which have hitherto been among 
the great mysteries of the vertebrate organization. For not only 
do the median eyes (C.E.) correspond in position in the two diagrams, 
and the infundibular tube (Inf.) and the ventricles of the brain 
(C.C.) correspond to the cesophagus (ces.) and the cephalic stomach 
(AL), as already fully discussed ; but even in the very place where the 
narrow cesophagus opened into the wider chamber of the pharynx 
(Ph.), there, in all the lower vertebrates, the narrow infundibular tube 
opens into the wider chamber of the membranous saccus vasculosus (sac. 
vase). This is the last portion of the membranous part of the tube of 
the central nervous system which has not received explanation in the 
previous chapters, and now it is seen how simple its explanation is, 



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PROSOMATIC SEGMENTS OF LIMULUS 



245 



M.obl. 




01f P' Hyp / En ! a 
cox.jl 




n,IV 




Pit. :?:3*T 

sac.i/asc 



n.JV 






~7^~j w " „, f . 




Fig. 10G. — Diagram of Sagittal Median Section through B, Eubypterus 
C, Larval Ammoc^etes ; D, Full-grown Ammoccetes. 



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246 THE ORIGIN OF VERTEBRATES 

how natural its presence — it represents the old pharyngeal chamber 
of the paheostracan ancestor. 

Next among the mysteries requiring explanation is the pituitary 
body, that strange glandular organ always found so closely attached 
to the brain in the infundibular region that when it is detached in 
taking out the brain it leaves the infundibular canal patent right into 
the Illrd ventricle. A comparison of the two diagrams indicates 
that such a glandular organ (Pit.), Fig. 106, C, was there because the 
coxal excretory glands (cox. gl.), Fig. 106, B, were in a similar 
position in the palreostracan ancestor — that, indeed, the pituitary 
body is the descendant of the coxal glands. 

Finally, the diagrams not only indicate how the mesosomatic 
appendage-nerves supplying in the one case the operculum and the 
respiratory appendages correspond to the respiratory group of nerves, 
VII., IX., X., supplying in the other case the thyroid, hyoid, and 
branchial segments, but also that a similar correspondence exists 
between the prosomatic appendage-nerves in the one case and the 
trigeminal nerve in the other ; a correspondence which supplies the 
reason why in the vertebrate a septum originally existed between an 
oral and respiratory chamber. 

Such a comparison, then, leads directly to the suggestion that the 
trigeminal nerve originally supplied the prosomatic appendages, such 
appendages being: 1. The metastoma, which has become in Ammo- 
ccetes the lower lip supplied by the velar or mandibular branch of the 
trigeminal nerve (7) ; 2. The ectognath, wliich has become the large 
median ventral tentacle, called by Rathke the tongue, supplied by the 
tongue nerve (6) ; 3. The endognaths, which have been reduced to 
tentacles and are supplied by the tentacular branch of the trigeminal 
nerve (2, 3, 4, 5). 

I have purposely put these two diagrams of the larval Ammo- 
ccetes and of Eurypterus before the minds of my readers at this early 
stage of my argument, so as to make what follows more understand- 
able. I propose now to consider fully each one of these suggestive 
comparisons, and to see whether or no they are in accordance with 
the results of modern research. 

In the first instance, the diagrams suggest that the trigeminal 
nerve originally supplied the prosomatic appendages of the palaeo- 
stracan ancestor, while the eye-muscle nerves supplied the body- 
muscles of the prosoma. 



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PROSOMATIC SEGMENTS OF LIMULUS 247 

As these appendages did not carry any vital organs such as 
branchiae, but were mainly locomotor and masticatory in function, it 
follows that their disappearance as such would be much more com- 
plete than that of the inesosomatic branchial appendages. Most 
probably, then, in the higher vertebrates no trace of such appendages 
might be left ; consequently the segmentation due to their presence 
would be very obscure, so that in this region the very reverse of what 
is found in the region of the vagus nerve would be the rule. There 
branchiomeric segmentation is especially evident, owing to the per- 
sistence of the branchial part of the branchial appendages; here, 
owing to the disappearance of the appendages, the segmentation is 
no longer brancliiomeric, but essentially mesomeric in consequence 
of the persistence of the somatic eye-muscles. 

In addition to the evidence of the appendages themselves, the 
number of prosomatic segments is well marked out in all the 
members of the scorpion group by the divisions of the central 
nervous system into well-defined neuromeres in accordance with the 
appendages, a segmentation the reminiscence of which may still 
persist after the appendages themselves have dwindled or disappeared. 
In accordance with this possibility we see that one of the most 
recent discoveries in favour of a number of segments in the head- 
region of the vertebrate is the discovery in the early embryo of a 
number of partial divisions in the brain-mass, forming a system of 
cephalic neuromeres which may well be the rudiments of the well- 
defined cephalic neuromeres of animals such as the scorpion. 

The Evidence of the Prosomatic Musculature. 

Even if the appendages as such become obscure, yet their muscles 
might remain and show evidence of their presence. The most per- 
sistent of all the appendage-muscles are the basal muscles which pass 
from coxa to carapace and are known by the name of tergo-coxal 
muscles. They are large, well marked, segmentally arranged muscles, 
dorso-ventral in direction, and, owing to their connecting the limb 
with the carapace, are likely to be retained even if the appendage 
dwindles away. 

The muscular system of Limulus and Scorpio has l>een investi- 
gated by Benham and Miss Beck under Lankester's direction, and the 
conclusions to which Lankester comes are these — 



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248 THE ORIGIN OF VERTEBRATES 

The simple musculature of the primitive animal from which both 
Limulus and the scorpions arose consisted of — 

1. A series of paired longitudinal dorsal muscles passing from 

tergite to tergite of each successive segment. 

2. A similar series of paired longitudinal ventral muscles. 

3. A pair of dorso- ventral muscles passing from tergite to 

sternite in each segment. 

4. A set of dorso-ventral muscles moving the coxa of each limb 

in its socket. 

5. A pair of veno-pericardial muscles in each segment. 

Of these groups of muscles, any one of which would indicate the 
number of segments, Groups 1 and 2 do not extend into the proso- 
matic region, and Group 5 extends only as far as the heart extends 
in the case of both Limulus and the Scorpion group ; so that we may 
safely conclude that in the Palaeostraca the evidence of somatic 
segmentation in the prosomatic region would be given, as far as the 
musculature is concerned, by the dorso-ventral somatic muscles 
(Group 3), and of segmentation due to the appendages by the 
dorso-ventral appendage musculature (Group 4). 

Therefore, if, as the evidence so far indicates, the vertebrate has 
arisen from a palaeostracan stock, we should expect to find that the 
musculature of the somatic segments in the region of the trigeminal 
nerve did not resemble the segmental muscles of the spinal region, 
was not, therefore, the continuation of the longitudinal musculature 
of the body, but was dorso-ventral in position, and that the muscula- 
ture of the splanchic segments resembled that of the vagus region, 
where, as pointed out in Chapter IV., the respiratory muscles arose 
from the dorso-ventral muscles of the mesosomatic appendages. 
This is, of course, exactly what is found for the muscles which move 
the lateral eyes of the vertebrate ; these muscles, innervated by the 
Illrd, IVth, and Vlth nerves, afford one of the main evidences of 
segmentation in this region, are always grouped in line with the 
somatic muscles of spinal segments, and yet cannot be classed as 
longitudinal muscles. They are dorso-ventral in direction, and yet 
belong to the somatic system ; they are exactly what one ought to 
find if they represent Group 3 — the dorso-ventral body-muscles of 
the prosomatic segments of the invertebrate ancestor. 

The interpretation of these muscles will be given immediately ; 
at present I want to pass in review all the different kinds of evidence 



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PROSOMATIC SEGMENTS OF LIMULUS 249 

of segmentation in this region afforded by the examination of the 
invertebrate, whether living or fossil, so as to see what clues are left 
if the evidence of appendages fails us. I will take in the first instance 
the evidence of segmentation afforded by the presence of the muscu- 
lature of Group 4, even when, as in the case of many fossils, no 
appendages have yet been found. In such animals as Mygale and 
Phrynus the prosomatic carapace is seen to be marked out into a 
series of elevations and depressions, and upon removing the carapace 
we see that these elevations correspond with and are due to the large 
tergo-coxal muscles of the appendages ; so that if such carapace alone 
were found fossilized we could say with certainty : this animal pos- 
sessed prosomatic appendages the number of which can be guessed 
with more or less certainty by these indications of segments on the 
carapace. 

In those forms, then, which are only known to us in the fossil 
condition, in which no prosomatic appendages have been found, but 
which possess, more or less clearly, radial markings on the prosomatic 
carapace resembling those of Phrynus or Mygale, such radial markings 
may be interpreted as due to the presence of prosomatic appendages, 
which are either entirely concealed by the prosomatic carapace or 
dorsal head-plate, or were of such a nature as not to have been 
capable of fossilization. 

The group of animals in question forms the great group of animals, 
chiefly extinct, classified by H. Woodward under the order of Mero- 
stomata. They are divided by him into the sub-order of Eurypterida, 
which includes— (1) Pterygotus, (2) Slimonia, (3) Stylonurus, (4) 
Eurypterus, (5) Adelophthalmus, (6) Bunodes, (7) Arthropleura, (8) 
Hemiaspis, (9) Exapinurus, (10) Pseudoniscus ; and the sub-order 
Xiphosura, which includes— (1) Belinurus, (2) Prestwichia, (3) 
Limulus. 

The evidence of the Xiphosura and of the Hemiaspida conclusively 
shows, in Woodward's opinion, that the Merostomata are closely 
related to the Trilobita, and the Hemiaspida especially are supposed 
to be intermediate between the trilobites and the king-crabs. They 
are characterized, as also Belinurus and Prestwichia, by the absence 
of any prosomatic appendages, so that in these cases, as is seen in 
Fig. 12 (p. 30), representing Bunodes lunula, found in the Eurypterus 
layer at Rootzikull, we have an animal somewhat resembling Limulus 
in which the prosomatic appendages have either dwindled away and are 



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250 



THE ORIGIN OF VERTEBRATES 

Cc. 




Fig. l(yj.—Phrynus Margine-Maculata. 
Cc, median eyes; le. y lateral eyes; glab., median plate over brain ; Fo., fovea. 



supr.on gi- 




ro. obUea) 



Fig. 108.— Vhrynics up. (?). Carapace bemoyed. 
cam., camerostome ; pi., plastron. 



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PROSOMATIC SEGMENTS OF LIMULUS 



251 



3/ 






completely hidden by the prosomatic carapace, or became so soft as 
not to be preserved in the fossilized condition. The appearance of 
the prosomatic carapace is, to my 
mind, suggestive of the presence of 
such appendages, for it is marked out 
radially, as is seen in the figure, in a 
manner resembling somewhat the mark- 
ings on the prosomatic carapace of 
Mygale or Phrynus ; the latter mark- 
ings, as already mentioned, are due to 
the aponeuroses between the tergo-coxal 
muscles of the prosomatic appendages 
which lie underneath and are attached 
to the carapace. 

A very similar radial marking is 
shown by Woodward in his picture of 
Hemiaspis limuloides, reproduced in Fig. 
109, found in the Lower Ludlow beds at 
Leintwardine. This species has yielded 
the most perfect specimens of the genus 
Hemiaspis, which is recognized as differ- 
ing from Bunodes by the possession of a 
telson. 

It is striking to find that similar 



Si 



' -van? M 



indications of segments have been found 



Fig . 10'J. ^Hemiaspis limn loides . 
(From Woodward.) 

gl., glabellum. 



on the dorsal surface of the head-region 

in many of the most ancient extinct fishes, as will be fully discussed 

later on. 



The Evidence of Ccelomic Cavities. 

In the head-region of the vertebrate, morphologists depend largely 
upon the embryonic divisions of the mesoderm for the estimation of 
the number of segments, and, therefore, upon the number of ccelomic 
cavities in this region, the walls of which give origin to the striated 
muscles of the head, so that the question of the number of segments 
depends very largely upon the origin of the muscles from the walls of 
these head-cavities. It is therefore interesting to examine whether a 
similar criterion of segmentation holds good in such a segmented 



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252 THE ORIGIN OF VERTEBRATES 

animal as Limulus, or in the members of the scorpion group, in which 
the number of segments are known definitely by the presence of the 
appendages. In Limulus we know, from the observations of Kishi- 
nouye, that a series of coelomic cavities are formed embryologically in 
the various segments of the mesosoma and prosoma, in a manner 
exceedingly similar to their mode of formation in the head-region of 
the vertebrate, and he has shown that in the mesosoma a separate 
coelomic cavity exists for each segment, so that just as the dorso-ventral 
somatic muscles are regularly segmentally arranged in this region, so 
are the coelomic cavities, and we should be right in our estimation 
of the number of segments in this region by the consideration of 
the numerical correspondence of these cavities with the mesomatic 
appendages. Similarly, in the vertebrate, we find every reason to 
believe that a single, separate head-cavity corresponds to each of 
the branchial segments in the opisthotic region, and therefore we 
should estimate rightly the number of segments by the division of 
the mesoderm in this region. 

In the prosomatic region of Limulus, the dorso-ventral muscles 
are not arranged with such absolute segmental regularity as in the 
mesosomatic region, and Kishinouye's observations show that the 
coelomic cavities in this region do not correspond absolutely with 
the number of prosomatic appendages. His words are : — 

A pair of coelomic cavities appears in every segment except the 
segments of the 2nd, 3rd, and 4th appendages, in which coelomic 
cavities do not appear at all. At least eleven pairs of these cavities 
are produced. The eleventh pair belongs to the seventh abdominal 
segment. 

The first pair of coelomic cavities is common to the cephalic lobe 
and the segment of the first appendage (i.e. the chelicera). 

The second coelomic cavity belongs to the segment of the fifth 
appendage. It is well developed. 

The ventral portion of the second coelomic cavity remains as the 
coxal gland. 

Consequently, if we were to estimate the number of segments in 
this region by the number of ccelomic cavities we should not judge 
rightly, for we should find only four cavities and seven appendages, 
as is seen in the following table : — 



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PROSOMATIC SEGMENTS OF LIMULVS 



253 







LIMULUS. 




VERTEBRATE. 


Segments. 


Appendages. 


Eurypterid appendages. 


Ccelomic 
cavities. 


Coelomic cavities. 


( 


1 


Chelieene or 1st 
locomotor. 


Chelieene 


1 


Anterior 


6 j 


2 


2nd locomotor 


j 






« / 


3 
4 


3rd 
4th 


>Endognaths 


2 


Premandibular 


\ 
■ji 1 


5 


5th 


I 


: 






6 
7 

8 
9 


6th 
Chilaria 

Operculum 
1st branchial 


Ectognath 
Metastoma 


3 1 

4 i 

1 
5 
6 


[Mandibular 


| 
"-5 1 

eS 1 


v ( Genital 
[Operculum J 1st bran- 
] \ chial 


[Hyoid 


a 1 


10 


2nd „ 


2nd branchial 


7 | 


1st branchial 


\ 

CC 1 


11 


3rd 


3rd 


8 ' 


2nd „ 


m j 


12 


4th 


4th 


9 


3rd 


* 


13 


5th 


5th 


10 


4th „ 


1 


14 


6th 




11 

1 





The second cavity would in reality represent four segments 
belonging to the 2nd, 3rd, 4th, 5th locomotor appendages, i.e. the 
very four segments which in the Eurypteridae are concentrated 
together to form the endognaths, and we should be justified in put- 
ting this interpretation on it, because, according to Kishinouye, its 
ventral portion forms the coxal gland, and, according to Lankester, the 
coxal gland sends prolongations into the coxa of the 2nd, 3rd, 4th, 
5th locomotor appendages. Similarly in the vertebrate, we find three 
head-cavities in the region which corresponds, on my theory, to the 
prosomatic region of Limulus, (1) the anterior cavity discovered by 
Miss Piatt, (2) the premandibular cavity, and (3) the mandibular 
cavity, which, if they corresponded with the prosomatic coelomic cavities 
of Limulus, would represent not three segments but seven segments, 
as follows: — the anterior oavity would correspond to the first cu'lomic 
cavity, i.e. the cavity of the cheliceral segments in both Limulus and 
the Eurypteridie ; the premandibular, to the second ccelomic cavity, 
representing, therefore, the 2nd, 3rd, 4th, 5th prosomatic segments in 
Limulus and the endognathal segments in the Eurypteridoe ; and the 
mandibular to the 3rd and 4th ccelomic cavities, representing the last 
locomotor and chilarial segments in Limulus, i.e. the ectognathal and 
metastomal segments in the Eurypteridie. 



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254 THE ORIGIN OF VERTEBRATES 

It is worthy of note that, in respect to their ccelomic cavities, as in 
the position and origin of their nerves in the central nervous system, 
the first pair of appendages, the chelicerce, retain a unique position, 
differing from the rest of the prosomatic appendages. 

In the table I have shown how the vertebrate ccelomic cavities 
may be compared with those of Limulus. The next question to con- 
sider is the evidence obtained by morphologists and anatomists as to 
the number of segments supplied by the trigeminal nerve-group ; this 
question will be considered in the next chapter. 



Summary. 

In Chapters IV. and V. I have dealt with the opisthotic segments of the 
vertebrate, including" therein the segments supplied by the facial nerve, and 
shown that they correspond to the mesosomatic segments of the palseostracan ; 
consequently the facial (VII.). glossopharyngeal (IX.), and vagus (X.) nerves 
originally supplied the branchial and opercular appendages. 

In tlds chapter the consideration of the pro-otic segments is commenced, 
that is, the segments supplied by the trigeminal (V.) and the eye-muscle nerves 
(III., IV., VI.). I have considered the Vlth nerve with the rest of the eye- 
muscle nerves for convenience' sake, though in reality it belongs to the same 
segment as the facial. Of these, that part of the trigeminal which innervates 
the muscles of mastication corresponds to the splanchnic segments, while the 
eye-muscle nerves belong to the corresponding somatic segments ; but the 
pro-otic segments of the vertebrate ought to correspond to the prosomatic 
segments of the invertebrate, just as the opisthotic correspond to the meso- 
somatic. Therefore the motor part of the trigeminal ought to supply muscles 
which originally moved the prosomatic appendages, while the eye-muscles ought 
to have belonged to the somatic part of the same segments. 

The first question considered is the number of segments which ought to be 
found in this region. In Limulus, the Eurypteridae, and the scorpions there are 
seven prosomatic segments which carry (1) the chelicerae, (2, 3, 4, 5) the four 
first locomotor appendages — the endognaths, (6) the large special appendage — 
the ectognath — and (7) the appendages, which in Limulus are known as the 
chilaria, and are small and insignificant, but in Eurypterus and other forms 
grow forwards, fuse together, and form a single median lip to an accessory oral 
chamber, which lip is known as the metastoma. Of these appendages the 
chelicerte and endognaths tend to dwindle away and become mere tentacles, 
while the large swimming ectognath and metastoma remain strong and 
vigorous. 

In this, the prosomatic region, the somatic segmentation is not characterized 
by the presence of the longitudinal muscle segments, for they do not extend 
into this head-region, but only by the presence of the segmental somatic ventro- 



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PROSOMATIC SEGMENTS OF LIMULUS 255 

dorsal muscles. Among the muscles of the appendages the system of large 
tergo-coxal muscles is especially apparent. 

From these considerations it follows that the number of segments in this 
region in the vertebrate ought to be seven ; that the musculature supplied by 
the trigeminal nerve ought to represent seven ventral or splanchnic segments, 
of which only the last two are likely to be conspicuous ; and that the musculature 
supplied by the eye-muscle nerves ought to be dorso- ventral in direction, which 
it is, and represent seven dorsal or somatic segments. 

A further peculiarity of this region, both in Limulus and the scorpions, 
is found in the excretory organs which are known by the name of coxal glands, 
because they extend into the basal joint, or coxa, of certain of the prosomatic limbs. 
The appendages so characterized are always the four endognaths, and it follows 
that if these four endognaths lose their locomotor power, become reduced in 
size, and concentrated together to form mere tentacles, then of necessity the 
coxal glands will be concentrated together, and tend to form a glandular mass 
in the region of the mouth ; in fact, take up a position corresponding to that 
of the pituitary body in vertebrates. 

Taking all these facts into consideration, it is possible to construct a drawing 
of a sagittal section through the head-region of Eurypterus, which will 
represent, with considerable probability, the arrangement of parts in that 
animal. This can be compared with the corresponding section through the 
head of Ammocoetes. 

Now, as pointed out in the last chapter, the early stage of Ammocoetes is 
remarkably different from the more advanced stage ; at that time the septum 
between the oral and respiratory chambers has not yet broken through, and the 
olfactory or nasal tube, known at this stage as the tube of the hypophysis, is 
directed ventrally, not dorsally. 

The comparison of the diagram of Eurypterus with that of the early stage 
of Ammocoetes is remarkably close, and immediately suggests not only that the 
single nose of the former is derived from the corresponding organ in the 
palaeostracan, but that the pituitary body is derived from the concentrated 
coxal glands, and the lower lip from the metastoma. The further working out 
of these homologies will be discussed in the next chapter. 

In addition to the evidence of segmentation afforded by the appendages, there 
are in this region, in Limulus and the scorpion group, three other criteria of 
segmentation available to us, if from any cause the evidence of appendages fails 
us. These are — 

1. The number of neuromeres are marked out in this region of the brain 
more or less plainly, especially in the young animal, just as they are also in 
the embryo of the vertebrate. 

2. The segmentation is represented here, just as in the mesosomatic region, 
by two sets of muscle- segments ; the one somatic, consisting of the segmentally 
arranged dorso-ventral muscles, the continuation of the group already discussed 
in connection with the mesosomatic segmentation, and the other appendicular 
characterized by the tergo-coxal muscles. These latter segmental muscles are 
especially valuable, for in such forms as My gale, Phrynus, etc., their presence 
is indicated externally by markings on the prosomatic carapace, and thus corre- 
sponding markings found on fossil carapaces or on dorsal head- shields can be 



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256 THE ORIGIN OF VERTEBRATES 

interpreted. These two sets of muscle-segments correspond in the vertebrate 
to the somatic and splanchnic segmentations. 

3. In the vertebrate the segmentation in this region is indicated by the 
coelomic or head-cavities, which are cavities formed in the mesoderm of the 
embryo, the walls of which give origin to the striated muscles of the head. In 
Limulus corresponding coeloniic cavities are found, which are directly comparable 
with those found in the vertebrate. 



V 



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CHAPTER VIII 

THE SEGMENTS BELONGING TO THE TRIGEMINAL 
NERVE-GROUP 

The prosomatic segments of the vertebrate. — Number of segments belonging 
to the trigeminal nerve-group. — History of cranial segments. — Eye- muscles 
and their nerves.— Comparison with the dorso-ventral somatic muscles of the 
scorpion. — Explanation of the oculomotor nerve and its group of muscles. 
— Explanation of the trochlearis nerve and its dorsal crossing. — Explana- 
tion of the abducens nerve. — Number of segments supplied by the 
trigeminal nerves. — Evidence of their motor nuclei. — Evidence of their 
sensory ganglia. — Summary. 

From the evidence given in the last chapter, combined with that 
given in Chapter IV., the probability of the theory that the trigeminal 
group of nerves of the vertebrate have been derived from the 
prosomatic group of nerves of the invertebrate can be put to the 
test by the answers to the following morphological and anatomical 
questions : — 

1. Do we find in the vertebrate two segmentations in this region 
corresponding to the two segmentations in the branchial region, i.e. 
a somatic or dorsal series of segments, and a splanchnic or ventral 
series of segments ? The latter would not be branchial, but rather 
of the nature of free tactile appendages ; so that it is useless to look 
for or talk about gill-slits, although such appendages, being serially 
homologous with the branchial mesosomatic appendages, would 
readily give rise to the conception of branchial segments. 

2. Is there morphological evidence that the trigeminal nerve is 
not the nerve belonging to a single segment, or even to two segments, 
but is really a concentration of at least six, probably seven, segmental 
nerves ? 

3. Is there morphological evidence that the oculomotor and 
trochlear nerves, which on all sides are regarded as belonging to 
the trigeminal segments, are not single nerves corresponding each 

s 



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258 THE ORIGIN OF VERTEBRATES 

to a single segment, but are the somatic motor roots belonging to 
the same segments as those to which the trigeminal supplies the 
splanchnic roots ? 

4. Do the mesoderm segments, which give origin to the eye- 
muscles, and therefore do the head-cavities of this region, correspond 
with the trigeminal segments? Considering the concentration of 
parts in this region and the difficulty already presented by the want 
of numerical agreement between the prosomatic appendages and the 
prosomatic ccelomic cavities in Limulus, it may very probably be 
difficult to determine the actual number of the mesoderm segments. 

5. Is there anatomical evidence that the ganglion of origin of the 
motor part of the trigeminal nerve is not a single ganglion, but a 
representative of many, probably seven ? 

6. Is there anatomical evidence that the ganglia of origin of the 
oculomotor and trochlear nerves represent many ganglia ? 

7. Is there any evidence that the organs originally supplied by 
the motor part of the trigeminal nerve are directly comparable with 
prosomatic appendages ? 

It is agreed on all sides that in this region of the head there is 
distinct evidence of double segmentation, the dorsal mesoderm segments 
giving origin to the eye-muscles, and the ventral segments to the 
musculature innervated by the trigeminal nerve. Originally, accord- 
ing to the scheme of van Wijhe, two segments only were recognized, 
the dorsal parts of which were innervated by the Illrd and IVth 
nerves respectively. Since his paper, the tendency has been to 
increase the number of segments in this region, as is seen in the 
following sketch, taken from Kabl, of the history of cranial 
segmentation. 

History of Cranial Segmentation. 

The first attempt to deal with this question was made by Goethe 
and Oken. They considered that the cranial skeleton was composed 
of a series of vertebrae, but as early as 1842 Vogt pointed out that 
only the occipital segments could be reduced to vertebrae. In 1869, 
Huxley showed that vertebrae were insufficient to explain the 
cranial segmentation, and that the nerves must be specially con- 
sidered. The olfactory and optic nerves he regarded as parts of 
the brain, not true segmental nerves ; the rest of the cranial nerves 



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SEGMENTS OF TRIGEMINAL NERVE-GROUP 259 

were segmental, with special reference to branchial arches and clefts, 
the facial, glossopharyngeal, and separate vagus branches supplying 
the walls of the various branchial pouches. In a similar manner, 
the supra- and infra-maxillary branches of the trigeminal were 
arranged on each side of the mouth, and the inner and outer twigs of 
the first (ophthalmic) branch of the trigeminal on each side of the 
orbito-nasal cleft, the trabecular and the supra-maxillary arches being 
those on each side of this cleft. Thus Huxley considered that there 
was evidence of a series of pairs of ventral arches belonging to the 
skull, viz. the trabecular and maxillary in front of the mouth, the 
mandibular, hyoid, and branchial arches behind, and that the Vth, 
Vllth, IXth, and Xth nerves were segmental in relation to these 
arches and clefts. Gegenbaur, in 1871 and 1872, considered that the 
branchial arches represented the lower arches of cranial vertebrae, 
and therefore corresponded to lower arches in the spinal region, 
i.e. the skull was composed of as many vertebrae as there are 
branchial arches. These vertebrae were confined to the notochordal 
part of the skull, the prechordal part having arisen secondarily from 
the vertebral part, while the number of vertebrae are at least nine, 
possibly more. The nerves which could be homologized with spinal 
nerves were, he thought, divisible into two great groups— (1) the 
trigeminal group, which included the eye-muscle nerves, the facial, 
and its dorsal branch, the auditory; (2) the vagus group, which 
included the glossopharyngeal and vagus. 

Such was the outcome of the purely comparative anatomical 
work of Huxley and Gegenbaur — work that has profoundly influenced 
all the views of segmentation up to the present day. 

Now came the investigations of the embryologists, of whom I 
will take, in the first instance, Balfour, whose observatigns on the 
embryology of the Selachians led him to the conclusion that besides 
the evidence of segmentation to be found in the cranial nerves and 
in the branchial clefts, further evidence was afforded by the existence 
of head-cavities, the walls of which formed muscles just as they do 
in the spinal region. He came to the conclusion that the first head- 
cavity belonged to one or more pre-oral segments, of which the nerves 
were the oculomotor, trochlearis, and possibly abducens ; while there 
were seven post-oral segments, each with its head-cavity and its 
visceral arch, of which the trigeminal, facial, glossopharyngeal, and 
the four parts of the vagus were the respective nerves. 



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260 THE ORIGIN OF VERTEBRATES 

Marshall, in 1882, considered that the cranial segments were all 
originally respiratory, and that all the segmental nerves are arranged 
uniformly with respect to a series of gill-clefts which have become 
modified anteriorly and have been lost, to a certain extent, pos- 
teriorly. He included the olfactory nerves among the segmental 
nerves, and looked upon the olfactory pit, the orbito-nasal lacrymal 
duct, the mouth, and the spiracle as all modified gill-slits, so that he 
reckoned three pre-oral and oral segments belonging to the 1st, Illrd, 
I Vth, and Vth nerves, and eight post-oral segments belonging respec- 
tively to the Vllth and Vlth nerves, and to the IXth nerve, and six 
segments belonging to the Xth nerve. He pointed out that muscles 
supplied by the oculomotor nerve develop from the outer wall of the 
first head-cavity ; not, however, the obliquus superior and recttvs 
cxterntts, the latter originating probably from the walls of the third 
cavity. 

In the same year, 1882, came van Wijhe's well-known paper, in 
which he showed that the mesoderm of the head in the selachian 
divided into two sets of segments, dorsal and ventral ; that the dorsal 
segments were continuous with the body-somites, and that the ven- 
tral segments formed the lateral plates of mesoblast between each of 
the visceral and branchial pouches. He concluded that the dorsal 
somites were originally nine in number, that each was supplied with 
a ventral nerve-root, in the same way as the somites in the trunk, 
and that to each one a visceral pouch corresponded, whose walls 
were supplied by the corresponding dorsal nerve-root ; of these nine 
segments, the ventral nerve-roots of the first three segments were 
respectively the oculomotor, trochlearis, and abducens nerves. The 
next three segments possessed no definable ventral root or muscles, 
and the seventh, eighth, and ninth segments possessed as ventral 
roots the hypoglossal nerve, with its muscular supply. The corre- 
sponding dorsal nerve-roots were the trigeminal, facial, auditory, 
glossopharyngeal and vagus nerves, the difference between cranial 
and spinal dorsal roots being that the former contain motor 
fibres. 

Ahlborn, in 1884, drew a sharp distinction between the segments 
of the mesoderm and those of the endoderm. The former segmenta- 
tion he called mesomeric, the latter branch iomeric. He considered 
the two segmentations to be independent, and concluded that the 
branchiomeric was secondary to the mesomeric, and therefore not of 



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SEGMENTS OF TRIGEMINAL NERVE-GROUP 26 1 

segmental value. As to the segments of the mesoderm in the head, 
the three hindmost or occipital in Petroniyzontidoe remain perma- 
nently, and correspond to the three last segments in the selachian head. 
Of the anterior mesoderm segments, he considered that there were 
originally six, and that there are six typical eye-muscles in all 
Craniota, which have been compressed into three segments, as in 
Selachia. 

Froriep (1885) showed in sheep-embryos and in chicks that the 
hypoglossal nerve belongs to three proto-vertebne posterior to the 
vagus region, which were true spinal segments. He therefore modified 
Gegenbaur's conceptions to this extent: that portion of the skull 
designated by Gegenbaur as vertebral must be divided into two parts 
— a hind or occipital region, which is clearly composed of modified 
vertebrae and is the region of the hypoglossal nerves, and a front 
region, extending from the oculomotor to the accessorius nerves, which 
is characterized segmentally by the formation of branchial arches, but 
in which there is no evidence that proto-vertebrie were ever formed. 
He therefore divides the head-skeleton into three parts — 

1. Gegenbaur's evertebral part — the region of the olfactory and 
optic nerves — which cannot be referred to any metameric segmen- 
tation. 

2. The pseudo-vertebral, pre-spinal, or branchial part, clearly 
shown to be segmented from the consideration of the nerves and 
branchial arches, but not referable to proto-vertebrae — the region of 
the trigeminal and vagus nerves. 

3. The vertebral spinal part — the region of the hypoglossal 
nerves. 

He further showed that the ganglia of the specially branchial 
nerves, the facial, glossopharyngeal, and vagus, are at one stage 
in connection with the epidermis, so that these parts of the epidermis 
represent sense-organs which do not develop ; these organs probably 
belonged to the lateral line system. As the connection takes place 
at the dorsal edge of the gill-slits, they may also be called rudimen- 
tary branchial sense-organs. 

Since this paper of Froriep's, it has been generally recognized, 
and Gegenbaur has accepted Froriep's view, that the three hindmost 
metameres, which distinctly show the characteristics of vertebra, 
l>elong to the spinal and not to the cranial region, so that the 
metameric segmentation of the cranial region proper has become 



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262 THE ORIGIN OF VERTEBRATES 

more and more associated with the branchial segmentation. Froriep's 
discovery of the rudimentary branchial sense-organs as a factor in 
the segmentation question has led Beard to the conclusion that the 
olfactory and auditory organs represent in a permanent form two 
of these rudimentary branchial sense-organs. He therefore includes 
both the olfactory and auditory nerves in his list of cranial segmental 
nerves, and makes eleven cranial branchial segments in front of the 
spinal segments represented by the hypoglossal. 

A still larger number of cranial segments is supposed to exist, 
according to the researches of Dohrn and Killian, in the embryos 
of Torpedo ocellata. The former, holding to the view that vertebrates 
arose from annelids, considered that the head was formed of a series 
of metameres, to each one of which a mesoderm -segment, a gill-arch, 
a gill-cleft, a segmental nerve and vessel belonged. He found in the 
front head-region of a Torpedo embryo, corresponding to van Wijhe's 
first four somites, no less than twelve to fifteen mesoderm segments, 
and concluded, therefore, that the eye-muscle nerves, especially the 
oculomotor, represented many segmental nerves, and were not the 
nerves of single segments ; so, also, that the inferior maxillary part of 
the trigeminal and the hyoid nerve of the facial are probably not 
single nerves, but a fusion of several. Killian comes to much the 
same conclusion as Dohrn, for he finds seventeen to eighteen separate 
mesoderm segments in the head, of which twelve belong to the tri- 
geminal and facial region. 

Since Rabl's paper, a number of papers have appeared, especially 
from America, dealing with yet another criterion of the original 
segmentation of the head, viz. a series of divisions of the central 
nervous system itself, which are seen at a very early stage of 
development, and are called neuromeres ; the divisions in the cranial 
region being known as encephalomeres, and those of the spinal region 
as myomeres. Locy's paper has especially brought these divisions 
into prominence as a factor in the question of segmentation. They 
are essentially segments of the epiblast and not of the mesoblast ; 
they are conspicuous in very early stages, and appear to be in 
relation with the cranial nerves, according to Locy. He recognizes 
in Squalus acanthias, in front of the spino-occipital region, fourteen 
pairs of such encephalomeres and a median unsegmented termination, 
which may represent one more pair fused in the middle line, making 
at least fifteen. He distributes these fifteen segments as follows : 



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SEGMENTS OF TRIGEMINAL NERVE-GROUP 263 

fore-brain three and unsegmented termination, mid-brain two, and 
hind-brain nine. 

Again, Kupffer, in bis recent papers on the embryology of Ammo- 
coetes, asserts that especial information as to the number of primitive 
segments is afforded by the appearance in the early stages of a series 
of epibranchial ganglia in connection with the cranial nerves, which 
remain permanently in the case of the vagus nerves, but disappear 
in the case of pro-otic nerves. He considers that the evidence points 
to the number of segments in the mid- and hind-brain region as 
being primitively fifteen, viz. six segments belonging to the tri- 
geminal and abducens group, three segments belonging respectively 
to the facial, auditory, and glossopharyngeal, and six to the vagus. 

From this sketch we see that the modern tendency is to make six 
segments at least out of the region of the trigeminal nerves rather 
than two. In this region, as already mentioned, the evidence of 
segmentation is based more clearly on the somatic than on the 
splanchnic segments. We ought, therefore, in the first place, to 
consider the teaching of the eye-muscles and their nerves and the 
ccelomic cavities in connection with them, and see whether the 
hypothesis that such muscles represent the original dorso-ventral 
somatic muscles of the pakeostracan ancestor is in harmony with 
and explains the facts of modern research. 

Eye-Muscles and their Nerves. 

The only universally recognized somatic nerves belonging to these 
segments which exist in the adult are the nerves to the eye-muscles, 
of which, according to van Wijhe, the oculomotor is the nerve of the 
1st segment, the trochlearis of the 2nd, and the abducens of the 3rd ; 
while the nerves and muscles belonging to the 4th and 5th segments, 
ix. the 2nd facial and glossopharyngeal segments respectively, show 
only the merest rudiments, and do not exist in the adult. One 
significant fact appears in this statement of van Wijhe, and is 
accepted by all those who follow him, viz. that the oculomotor nerve 
has equal segmental value with the trochlearis and the abducens, 
although it supplies a number of muscles, each of which, on the face 
of it, has the same anatomical value as the superior oblique or 
external rectus. Dohrn alone, as far as I know, as already pointed 
out, insists upon the multiple character of the oculomotor nerve. 



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264 THE ORIGIN OF VERTEBRATES 

As far as the anatomist is concerned, the evidence is becoming 
clearer and clearer that the nucleus of the Illrd nerve is a composite 
ganglion composed of a number of nuclei, each similar to that of the 
trochlearis, so that if the trochlearis nucleus is a segmental motor 
nucleus, then the oculomotor nucleus is a combined nucleus belong- 
ing to at least four segmental nerves, each of which has the same 
value as that of the trochlearis. 

The investigations of a number of anatomists, among whom may 
be mentioned Gudden, Obersteiner, Edinger, Kdlliker, Gehuchten, 
all lead directly to the conclusion that this oculomotor nucleus is 
composed of a number of separate nuclei, of which the most anterior 
as also the Edinger- Westphal nucleus contains small cells, while the 
others contain large cells. Thus Edinger divides the origin of the 
oculomotor nerve into a small-celled anterior part and a larger 
posterior part, of which the cells are larger and distinctly arranged 
in three groups— (1) dorsal, (2) ventral, and (3) median. Between 
the anterior and posterior groups lies the Edinger- Westphal nucleus, 
which is small-celled ; naturally, the large-celled group is that which 
gives origin to the motor nerves of the eye-muscles, the small-celled 
being possibly concerned with the motor nerves of the pupillary and 
ciliary muscles. I may mention that Kolliker considers that the 
anterior lateral nucleus has nothing to do with the oculomotor nerve, 
but is a group of cells in which the fibres of the posterior longi- 
tudinal bundle and of the deep part of the posterior commissure 
terminate. 

These conclusions of Edinger are the outcome of work done in 
his laboratory by Perlia, who says that in new-born animals the 
nucleus of origin of the oculomotor nerve is made up of a number ■ 
of groups quite distinct from each other, each group being of the 
same character as that of the trochlearis. He finds the same 
arrangement in various mammals and birds. Further, he finds that 
some of the fibres arise from the nucleus of the opposite side, thus 
crossing, as in the trochlearis ; these crossing fibres belong to the 
most posterior of the dorsal group of nuclei, i.e. to the nerve to the 
inferior oblique muscle. 

The evidence, therefore, points to the conclusion that the oculo- 
motor nucleus is a multiple nucleus, each part of which gives origin 
to one of the nerves of one of the eye-muscles. 

Edinger says that such an array of clinical observations exists, 



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SEGMENTS OF TRTGEAflNAL NERVE-GROUP 265 

and of facts derived from post-mortem dissections, that one may 
venture to designate the portion of the nucleus from which the 
innervation of each individual ocular muscle comes. He gives Starr's 
table, the latest of these numerous attempts, begun by Pick. Accord- 
ing to Starr, the nuclei of the nerves to the individual muscles are 
arranged from before backward, thus — 

m. sphincter iridis. m. ciliaris. 

m. levator palpebrcc. m. rectus internus. 

m. rectus superior. m. rectus inferior. 

m. obliquus inferior. 
Further, the evidence of the well-known physiological experi- 
ments of Hensen and Volckers that the terminal branches of the 
oculomotor nerve arise from a series of segments of the nucleus, 
arranged more or less one behind the other in a longitudinal row, 
leads them to the conclusion that the nuclei of origin are arranged as 
follows, proceeding from head to tail : — 



Nearest brain. 


1. 
2. 
3. 


m. ciliaris. 

m. sphincter iridis. 

m. rectus internus. 




4. 
5. 
6. 


m. rectus mperior. 
m. levator palpebrcc. 
m. rectus inferior. 


Most posterior. 


7. 


m. obliquus inferioi\ 



It is instructive to compare this arrangement of Hensen and 
Volckers with the arrangement of the origin of these muscles from 
the premandibular cavity as given by Miss Piatt. 

Thus she states that the most posterior part of the premandibular 
cavity is cut off so as to form a separate cavity, resembling, except 
in position, the anterior cavity ; this separate, most posterior part 
gives origin to the inferior oblique muscle. She then goes on to 
describe how the dorsal wall of the remainder of the premandibular 
cavity becomes thickened, to form posteriorly the rudiment of the 
inferior rectus and anteriorly the rudiments of the superior and 
internal recti, a slight depression in the wall of the cavity separating 
these rudiments. The internal rectus is the more median of the 
two anterior muscles. In other words, her evidence points not only 
to a fusion of somites to form the premandibular cavity, but also 
to the arrangement of these somites as follows, from head to tail : 
(1) internal rectus, (2) superior rectus, (3) inferior rectus, (4) inferior 



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266 THE ORIGIN OF VERTEBRATES 

oblique — an order precisely the same as that of Hensen and Volckers, 
and of Starr. 

I conclude, from the agreement between the anatomical, physio- 
logical, and morphological evidence, that the Illrd and IVth nerves 
contain the motor somatic nerves belonging to the same segments as 
the motor trigeminal, in other words, to the prosomatic segments, so 
that the eye-muscles, innervated by III. and IV., represent segmental 
muscles belonging to the prosoma. Further, I conclude that originally 
there were seven prosomatic segments, the first of which is repre- 
sented by the anterior cavity described by Miss Piatt, and does not 
form any permanent muscles ; that the next four belong to the pre- 
mandibular cavity, and the muscles formed are the superior rectus, 
internal rectus, inferior rectus, and inferior oblique ; and that the last 
two belong to the mandibular cavity, the muscles formed being Miss 
Piatt's mandibular muscle and the superior oblique. It is, to say the 
least of it, a striking coincidence that such an arrangement of the 
ccelomic cavities as here given should be so closely mimicked by 
the arrangement in the prosomatic region of Limulus as already 
mentioned ; it suggests inevitably that the head-cavities of the verte- 
brate are nothing more than the prosomatic and mesosomatic 
segmental coplomic cavities, as found in animals such as Limulus. 
In the table on p. 253, 1 have inserted the segments in the vertebrate 
for comparison with those of Limulus. 

Before we can come to any conclusion as to the original position 
of these eye-muscles, it is necessary to consider the Vlth nerve and 
the external rectus muscle. This nerve and this muscle belong to 
van Wijhe's 4th segment. The muscle is, therefore, the somatic 
segmental muscle belonging to the same segment as the facial and is, 
in fact, a segmental muscle belonging not to the prosoma, but to the 
mesosoma. Neal comes to the conclusion that the existing abducens 
is the only root which remains permanent among a whole series of 
corresponding ventral roots belonging to the opisthotic segments, and 
further points out that the external rectus was originally an opis- 
thotic muscle which has taken up a pro-otic position, or, translating 
this statement into the language of Limulus, etc., it is a mesosomatic 
muscle which has taken up a prosomatic position. 

There is, however, another muscle — the Retractor oculi — belonging 
to the same group which is innervated by the Vlth nerve. Quite 
recently Edgeworth has shown that in birds and reptiles this muscle 



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SEGMENTS OF TRIGEMINAL NERVE-GROUP 267 

belongs to the hyoid segment ; so that in this respect also the hyoid 
segment proclaims its double nature. 

With respect to the external rectus muscle, Miss Piatt has shown 
that the mandibular muscle is formed close alongside the external rectus, 
so that the two are in close relationship as long as the former exists. 

Further, as already mentioned, the eye-muscles in Ammoccetes 
must be considered by themselves ; they do not belong in structure 
or position to the longitudinal somatic muscles innervated by the 
spinal nerves ; their structure is not the same as that of the tubular 
constrictor or branchial muscles, but resembles that structure some- 
what ; their position is dorso- ventral rather than longitudinal ; they 
may be looked upon as a primitive type of somatic muscles seg- 
mentally arranged, the direction of which was dorso-ventral. 

Anderson also has shown that the time of medullation of the 
nerves supplying these muscles is much earlier than that of the 
nerves belonging to the somatic trunk-muscles, their medullation 
taking place at the same time as that of the motor nerves supplying 
the striated visceral muscles ; and Sherrington has observed that 
these muscles do not possess muscle-spindles, while all somatic 
trunk-muscles do. Both these observations are strong confirmation 
of the view that the eye-muscles must be classified in a different 
category to the ordinary somatic trunk muscle group. 

What, then, is the interpretation of these various embryological 
and anatomical facts ? 

Eemembering the tripartite division of each segmental nerve-group 
in Limulus into (1) dorsal or sensory somatic nerve, (2) appendage- 
nerve, and (3) ventral somatic nerve, I venture to suggest that the 
three nerves— the ocufomotorius, the trochlearis, and the abducens 
— represent the ventral somatic nerves of the prosoma, and partly 
also of the mesosoma ; that they are nerves, therefore, which may 
have originally contained sensory fibres, and which still contain the 
sensory fibres of the eye-muscles themselves, as stated by Sherrington. 
According to this suggestion, the eye-muscles are the sole survivors 
of the segmental dorso-ventral somatic muscles, so characteristic of 
the group from which I imagine the vertebrates to have sprung. In 
the me808omatic region the dorso-ventral muscles which were retained 
were those of the appendages and not of the mesosoma itself, because 
the presumed ancestor breathed after the fashion of the water- 
breathing Limulus, by means of the dorso-ventral muscles of its 



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268 THE ORIGIN OF VERTEBRATES 

branchial appendages, and not after the fashion of the air-breathing 
scorpion, by means of the dorso-ventral muscles of the mesosoma. 
The only mesosomatic dorso-ventral muscles which were retained 
were those of the foremost mesosomatic segments, i.e. those supplied by 
the Vlth nerve, which were preserved owing to their having taken on 
a prosomatic position and become utilized to assist in the movements 
of the lateral eyes. 

Let us turn now to the consideration of the corresponding muscu- 
lature in Limulus and in the scorpion group. These muscles con- 
stitute the markedly segmental muscles to which I have given the 
name ' dorso-ventral somatic muscles.' They are most markedly 
segmental in the mesosomatic region, both in Limulu3 and in Scorpio, 
each mesosomatic segment possessing a single pair of these vertical 
mesosomatic muscles, as Benham calls them (cf. Fig. 58 (Dv.)). In 
the prosomatic region the corresponding muscles are not so clearly 
defined in Limulus; they are apparently attached to the plastron 
forming the group of plastro-tergal muscles. From Benham's descrip- 
tion it is sufficiently evident that they formed originally a single pair 
to each prosomatic segment. 

In Scorpio, according to Miss Beck, the dorso-ventral prosomatic 
muscles are situated near the middle line on each side and form the 
following well-marked series of pairs of muscles, shown in Fig. 110, A, 
taken from her paper, and thus described by her : — 

1. The dorso-cheliceral-sternal muscle (61) is the most anterior 
of the dorso-ventral muscles. It is very small, and is attached to the 
carapace near the median line anteriorly to the central eyes. 

2. The median dorso-preoral-entosclerite muscle (62) is a large 
muscle, between which and its fellow of the opposite side the eyes are 
situated. It is attached dorsally to the carapace and ventrally to the 
pre-oral entosclerite. 

3. The anterior dorso-plastron muscle (63) is attached dorsally 
to the carapace in the middle line, being joined to its fellow of the 
opposite side. They separate, and are attached ventrally to the 
plastron. Through the arch thus formed the alimentary canal and 
the dorsal vessel pass. 

4. The median dorso-plastron muscle (64) is attached dorsally to 
the posterior part of the carapace. It runs forward on the anterior 
surface of the posterior flap of the plastron to the body of the plastron, 
to which it is attached. 



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SEGMENTS OF TRIGEMINAL NERVE-GROUP 269 




Dorso - ventral Muscles on 
Carapace of Scorpion. (From 
Miss Beck.) 



B. 

Similar Muscles on Carapace 
of eurypterus. 



C. 

Similar Muscles on Hkad- 
Shield of a Cephalaspid. 

l.c. y lateral eyes ; c.e., central 
eyes ; Fro., narial opening. 

62-65 refer to Miss Beck's cata- 
logue of the scorpion muscles. 



Fig. 110. 



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270 THE ORIGIN OF VERTEBRATES 

To these may be added, owing to its attachment to the plastron, 

5. The posterior dorso-plastron muscle (65). This is the first of 
the dorso-ventral muscles attached to the mesosomatic tergites, being 
attached to the tergite of the first segment of the mesosoma. 

This muscle is of interest, in connection with the prosomatic 
dorso-ventral muscles, because it is attached to the plastron, and runs 
a course in close contact with the muscle (64), the two muscles being 
attached dorsally close together, on each side of the middle line, the 
one at the very posterior edge of the prosomatic carapace, and the 
other at the very anterior edge of the mesosomatic carapace. 

Taking these muscles separately into consideration, it may be 
remarked with respect to (61) that the cheliceral segment in its 
paired dorso-ventral muscles, as in its tergo-coxal muscles, takes 
up a separate position isolated from the rest of the prosomatic 
segments. 

Next comes (62) the median dorso-preoral-entosclerite muscle, 
which is strikingly different from all the other dorso-ventral muscles 
in its large size and the extent of its attachment to the dorsal cara- 
pace, according to Miss Beck's figures. The reason of its large size 
is clearly seen upon dissection of the muscles in fiuthus, for I find 
that, strictly speaking, it is not a single muscle, but is composed of 
a series of muscle-bundles, separated from each other by connective 
tissue. There are certainly three separate muscles included in this 
large muscle, which are attached in a distinct series along the pre-oral 
entosclerite, and present the appearance given in Fig. 110, A, at their 
attachment to the prosomatic carapace. Of this muscle-group the 
most anterior and the most posterior bundle are distinctly separate 
muscles ; I am not, however, clear whether the middle bundle 
represents one or two muscles. 

This division of Miss Beck's muscle (62) into three or four 
muscles brings the prosomatic region of the scorpion into line with 
the mesosomatic, and enables us to feel sure that a single pair of 
dorso-ventral somatic muscles belongs to each prosomatic segment 
just as to each mesosomatic, and, conversely, that each such single 
pair of muscles possesses segmental value in this region as much as 
in the mesosomatic. 

It is very striking to see how in all the Scorpionidae, in which the 
two median eyes are the principal eyes, this muscle group (62) on 
the two sides closely surrounds these two eyes, so that with a fixed 



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SEGMENTS OF TRIGEMINAL NERVE-GROUP 27 1 

pre-oral entosclerite, a slight movement of the eyes, laterally or 
anteriorly, owing to the flexibility of the carapace, might result as 
the consequence of their contraction. But this cannot be the main 
object of these muscles. The pre-oral entosclerite is firmly fixed to 
the camerostome, as is seen in Fig. 94, pr. ent. f so that the main 
object of these muscles is, as Huxley has pointed out, the movement 
of this organ. 

In order to avoid repetition of the long name given to this muscle 
group (62) by Miss Beck, because of their position, and for other 
reasons which will appear in the sequel, I will call this group of 
muscles the group of recti muscles. These recti muscles belong 
clearly to the segments posterior to the first prosomatic or cheliceral 
segment, and represent certainly three, probably four, of these 
segments, i.e. belong to the segments corresponding to the second, 
third, fourth, and fifth prosomatic locomotor appendages — the endo- 
gnaths of the old Eurypterids. 

The next pair of muscles is the pair of anterior dorso-plastron 
muscles (63). This muscle-pair evidently belongs to a segment pos- 
terior to the segments represented by the group already discussed, 
and belongs, therefore, in all probability to the same segment as the 
sixth pair of prosomatic appendages — the ectognaths of the old 
Eurypterids. This can be settled by considering either the nerve- 
supply or the embryological development. In the Eurypteridae it 
seems most highly probable that the dorso-ventral muscles of each 
half of the segments belonging to the endognaths should be compressed 
together and separate from the dorso-ventral muscle belonging to the 
ectognathal segment, on account of the evident concentration and small 
size of the endognathal segments in contradistinction to the separate- 
ness and large size of the ectognathal segment. 

The striking peculiarity of this muscle-pair, which distinguishes it 
from all other muscles in the scorpion, is the common attachment of 
the muscles of the two sides in the mid-dorsal line, so that the pair 
of muscles forms an arch through which the alimentary canal and 
dorsal blood-vessel pass. 

The same dorso-ventral muscles are present in Phrynus, and in 
this animal the fibres of this pair of muscles (63) actually interlace 
before the attachment to the prosomatic carapace, so that the attach- 
ment of the muscle on each side overpasses the mid-dorsal line, and 
a true crossing occurs. In Fig. 108 the position of this pair of 



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272 THE ORIGIN OF VERTEBRATES 

muscles is shown just posteriorly to the brain-mass. This muscle 
I will call the oblique muscle. 

Finally we come to the muscles (64) and (65), the median and 
posterior dorso-plastron muscles, which run close together. Both 
muscles are attached to the plastron, and, therefore, to that extent 
belong to the prosomatic region ; they are attached dorsally close to 
the junction of the prosoma and mesosoma. This position of the 
first mesosomatic dorso-ventral muscle belonging to the opercular 
segment may be compared with the position of the first mesosomatic 
dorso-ventral muscle in Limulus which has become attached to the 
prosomatic carapace ; in both cases we see an indication that the 
foremost pair of mesosomatic dorso-ventral somatic muscles tend to 
take up a prosomatic position. 

As to the pair of small muscles (64), I believe that they repre- 
sent the dorso-ventral muscles of the seventh prosomatic segment 
(if the pair of muscles (63) belongs to the segment of the sixth loco- 
motor prosomatic appendages), i.e. they belong to the chilarial 
segment or metastoma. 

I desire to draw especial attention to the fact that the dorso- 
ventral muscle (64), which represents the seventh segment, always 
runs close alongside the dorso-ventral muscle (65), which represents 
the first mesosomatic or opercular segment. 

The comparison, then, of these two sets of facts leads to the 
following conclusions : — 

The foremost prosomatic or trigeminal segment stood separate 
and apart, being situated most anteriorly ; the musculature of this 
segment does not develop, so that the only evidence of its presence 
is given by the anterior coelomic cavity. This corresponds, according 
to my scheme, with the first or anterior ccelomic cavity of Limulus, 
and therefore represents, as far as the prosomatic appendages are 
concerned, the first prosomatic appendage-pair, or the chelicene ; the 
appendage-muscles being the muscles of the chelicerse, and the 
dorso-ventral somatic muscles the pair of dorso-cheliceral sternal 
muscles (61) in the scorpion. Both these sets of muscles, therefore, 
dwindle and disappear in the vertebrate. 

Then came four segments fused together to form the preman- 
dibular segment, the characteristic of which is the apparent non- 
formation of any permanent musculature from the ventral mesoderm- 
segments, and the formation of the eye-muscles innervated by the 



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SEGMENTS OF TRIGEMINAL NERVE-GROUP 273 

oculomotor nerve from the dorsal mesoderm segments. These four 
segments have been so fused together that van Wijhe looked upon 
them as a single segment, and the premandibular cavity as the cavity 
of a single segment. They represent, according to my scheme, the 
segments belonging to the endognaths, i.e. the second, third, fourth, 
fifth pairs of prosomatic appendages ; the premandibular cavity, there- 
fore, represents the second ccelomic cavity in Limulus, which, accord- 
ing to Kishinouye, is the sole representative of the ccelomic cavities 
of the second, third, fourth, fifth prosomatic segments. The muscles 
derived from the ventral mesoderm-segments represent the muscles 
of these appendages, which therefore dwindle and disappear in the 
vertebrate, with the possible exception of the muscles innervated by 
the descending root of the trigeminal. The muscles derived from 
the dorsal mesoderm-segments, i.e. the eye-muscles supplied by the 
oculomotor nerve, represent the dorso-ventral somatic muscles of these 
four segments, muscles which are represented in the scorpion by the 
recti group of muscles, i.e. the median dorso-preoral-entosclerite 
muscles (62). 

Then came two segments, the mandibular, in which muscles are 
formed both from the ventral and from the dorsal mesoderm-segments. 
From the former arose the main mass of muscles innervated by 
the motor root of the trigeminal, from the latter the superior oblique 
muscle and the mandibular muscle of Miss Piatt, of which the former 
alone survives in the adult condition. These two segments are looked 
upon as a single segment by van Wijhe, of which the mandibular 
cavity is the ccelomio cavity. They represent, according to my 
scheme, the segments belonging to the sixth pair of prosomatic 
appendages or ectognaths, and the seventh pair, i.e. the chilaria or 
metastoma. 

The first part, then, of the mandibular cavity represents the third 
ccelomic cavity in Limulus and the muscles derived from the ventral 
mesoderm, in all probability the muscles of the tongue in the 
lamprey (cf. Chap. IX.), which represents the ectognaths or sixth 
pair of prosomatic appendages, while the muscles derived from the 
dorsal mesoderm, i.e. the superior oblique muscles, represent the 
dorso-ventral somatic muscles of this segment, muscles which are 
represented in the scorpion group by the pair of anterior dorso- 
plastron or oblique muscles (G3). 

The second part of the mandibular cavity represents the 4th 

T 



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274 THE ORIGIN OF VERTEBRATES 

coelomic cavity in Liuiulus and the muscles derived from the ventral 
mesoderm, in all probability the muscles of the lower lip in the 
lamprey (cf. Chap. IX.), which represents the metastoma; while the 
muscles derived from the dorsal mesoderm, i.e. Miss Piatt's pair of 
mandibular muscles, represent the dorso-ventral somatic muscles of 
this segment, muscles which are represented in the scorpion group 
by the pair of median dorso-plastron muscles (64). 

In connection with this last pair of muscles we find that the 
external rectus in the vertebrate represents the first dorso-ventral 
mesosomatic muscle in the scorpion, i.e. the posterior dorso-plastron 
muscle (65), and, as already mentioned (p. 267), that it always lies 
closely alongside the mandibular muscle, just as in the scorpion group 
muscle (65) always lies alongside muscle (64). 

In the invertebrate as well as in the vertebrate this muscle is a 
mesosomatic muscle which has taken up a prosomatic position. 

The question naturally arises, what explanation can be given of 
the fact that these dorso-ventral muscles attached on each side 
of the mid-dorsal line to the prosomatic carapace became converted 
into the muscles moving the eyeballs of the two lateral eyes ? An 
explanation which must take into account not only the isolated posi- 
tion of the abducens nerve, but also the extraordinary course of the 
trochlearis. The natural and straightforward answer to this question 
appears to me quite satisfactory, and I therefore venture to commend 
it to my readers. 

I have argued the case out to myself as follows : The lateral eyes 
must have been originally situated externally to the group of muscles 
innervated by the oculomotor nerve, for a sheet of muscle representing 
the superior internal and inferior rectus muscles could only wrap 
round the internal surface of each lateral eye; i.e. the arrangement 
of the muscle-sheet, as in the scorpion, about two median eyes, is in 
the wrong position, for if those two eyes, which are the main eyes in 
the scorpion, were to move outwards to become two lateral eyes, then 
such a muscle-group would form a superior external and inferior rectus 
group. The evidence, however, of Eurypterus and similar forms is 
to the effect that the lateral eyes became big and the median eyes 
insignificant and degenerate. If, then, with the degeneration of the 
one and the increasing importance of the other, these lateral eyes 
came near the middle line, then the muscular group (62), which I 
have called the recti group, would naturally be pressed into their 



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SEGMENTS OF TRIGEMINAL NERVE-GROUP 



275 



.le 



service, and would form an internal and not an external group of 
eye-inuscles. 

In Fig. 110, A, taken from Miss Beck's paper, I have shown the 
relative position of the eyes and the segmental dorso-ventral pro- 
somatic muscles on the carapace of the scorpion. In Fig. 110, B, I 
have drawn the prosomatic carapace of Eurypterw Sconleri, taken 
from Woodward's paper, with the eyes as represented there ; in this 
I have inserted the segmental dorso-ventral muscles as met with in 
the scorpion, thereby demonstrating how, with the degeneration of 
the median eyes and the large size of the lateral eyes, the recti 
muscles of the scorpion would approach the position of an internal 
recti group to the lateral eyes, and so give origin to the group of 
muscles innervated by the oculomotor 
nerve. In the Eurypterus these large 
eyes are large single eyes, not separate 
ocelli, as in the scorpion. 

All, then, that is required is that in 
the first formed fishes, which still pos- 
sessed the dorso-ventral muscles of their 
Eurypterid ancestors, the lateral eyes 
should be the important organs of sight, 
large and near the mid-dorsal line. Such, 
indeed, is found to be the case. In 
amongst the masses of Eurypterids found 
in the upper Silurian deposits at Oesel, as 
described by Rohon, numbers of the most 
ancient forms of fish are found belonging 
to the genera Thyestes and Tremataspis. 
head-shields of these fishes is shown in Fig. 14, which represents 
the dorsal head-shield of Thyestes verrucosus, and Fig. Ill that of 
Tremataspis Mickwitzi. They show how the two lateral eyes were 
situated close on each side of the mid-dorsal line in these Eurypterus- 
like fishes, in the very position where they must have been if the 
eye-muscles were derived from the dorso-ventral somatic muscles of 
a Eurypterid ancestor. 

In Lankester's words, one of the characteristics of the Osteostraci 
(Cephalaspis, Auchenaspis, etc.), as distinguished from the Hetero- 
straci (Pteraspis), are the large orbits placed near the centre of the 
shield. The apparent exception of Thyestes mentioned by him is no 




— S 



Occ 



Fig. 111. — Dorsal Head- 
Shield op Tremataspis 
Michwitzi. (From Rohon.) 

Fro., narial opening; I.e., late- 
ral eyes ; gl., glabellum plate 
over brain; Occ., occipital 
spine. 

The nature of the dorsal 



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276 THE ORIGIN OF VERTEBRATES 

exception, for orbits of the same character have since been discovered, 
as is seen in Rohon's figure (Fig. 14). In Fig. 110, C, I give an 
outline of the frontal part of the head-shield of a Cephalaspid, in 
which I have drawn the eye-muscles as in the other two figures. 

Although all the members of the Osteostraci possess large lateral 
eyes towards the centre of the head-shield, the other group of ancient 
fishes, the Heterostraci, are characterized by the presence of lateral 
eyes far apart, situated on the margin of the head-shield on each 
side (cf. Fig. 142, 0, p. 350). 

So, also, on the invertebrate side, the lateral eyes of Pterygotus and 
Slimonia are situated on the margin of the prosomatic carapace, while 
those of Eurypterus and Stylonurus are situated much nearer the 
middle line of the prosomatic carapace. 

Next comes the question of the superior oblique muscle and the 
trochlearis nerve. Why does this nerve (ii.IV. in Fig. 106, C and D) 
alone of all the nerves in the body take the peculiar position it 
always does take? The only suggestion that I know of which 
sounds reasonable and worth consideration is that put forward by 
Fiirbringer, which is an elaboration of the original suggestion of 
Hoffmann. Hoffmann suggested in 1889 that the trochlearis nerve 
represented originally a nerve for a protecting organ of the pineal 
eye, which became secondarily a motor nerve for the lateral eye as 
the pineal eye degenerated. Fiirbringer differs from Hoffmann in 
that he considers that the nerve was originally a motor nerve, and 
was not transformed from sensory to motor, yet thinks Hoffmann's 
suggestion is in the right direction. 

He points out that the crossing of the trochlearis is not a crossing 
of fibres between two centres in the central nervous system, but may 
be explained by the shifting of the peripheral organ, i.e. the muscle, 
from one side to the other, and the nerve following this shift. Con- 
sequently, says Fiirbringer, the course of the nerve indicates the 
original position of the muscle, and therefore he imagines that the 
ancestor of the superior oblique muscle was a muscle the fibres of 
which were attached in the mid-dorsal line, and interlaced with those 
of the other side, the two muscles thus forming an arch through 
which the nervous system with its central canal passed. Then, for the 
sake of getting a more efficient pull, the crossing muscle-fibres became 
more definitely attached to the opposite side of the middle line, and 
finally obtained a new attachment on the opposite side, with the 



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SEGMENTS OF TRIGEMINAL NERVE-GROUP 277 

obliteration of the muscular arch ; the nerve on each side, following 
the shifts of the muscle, naturally took up the position of the original 
muscular arch, and so formed the trochlear nerve, with its dorsal 
crossing. This explanation of Furbringer's was associated by him 
with movements of the median pineal eyes, the length of their nerve, 
according to him, even yet indicating their previous mobility. This 
assumption is not, it seems to me, necessary. The length of the nerve 
is certainly no indication of mobility, for in Limulus and the scorpion 
group the nerve to each median eye is remarkably long, yet these 
eyes are immovably fixed in the carapace. All that is required is a 
pair of dorso-ventral muscles belonging to the segment immediately 
following the group of segments represented by the oculomotor nerves, 
the fibres of which should cross the mid-dorsal line at their attach- 
ment ; for, seeing that the lateral eyes were originally so near this 
position, it follows that such muscles might form part of the muscular 
group belonging to the lateral eye without having previously moved 
the pineal eyes. In fact, Furbringer's explanation requires as starting- 
point that the pair of muscles which ultimately become the superior 
oblique should have the exact position of the pair of dorso-ventral 
muscles in the scorpion, called by Miss Beck the anterior dorso- 
plastron muscles (63), which I have named the oblique muscles. 
Here, and here only, do we find an interlacement, across the mid- 
dorsal line, of the fibres of attachment of the muscles on the two sides, 
in consequence of which this pair of muscles is described by her as 
forming an arch encircling the alimentary canal and dorsal vessel. 
If, then, as I have previously argued, the primitive plastron formed a 
pair of trabecule, and the nervous system grew round the alimentary 
canal, such an arch would encircle the tubular central nervous system 
of the vertebrate. 

Still more striking is this pair of muscles (63) in Phrynus (Fig. 
108), where we see how the arch formed by them almost touches 
the posterior extremity of the supra-oesophageal brain-mass, crossing, 
therefore, over the beginning of the stomach region of the animal. 
The angle formed by the arch is much more obtuse than that formed 
in Scorpio, so that an actual crossing of the muscle-fibres has taken 
place at the point of attachment to the carapace. Also, only the part 
nearest the carapace is muscular, the rest forming a long tendinous 
prolongation of the plastron wall (the primordial cranium), as seen in 
the figure. 



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2 7 8 



THE ORIGIN OF VERTEBRATES 



This muscle-pair is, as it should be, the pair of dorso-ventral 
muscles belonging to the segment immediately following on the 
group of segments represented by the recti muscles, i.e. according 
to previous argument, the segment belonging to the sixth pair of 
locomotor appendages or ectognaths; a muscle, therefore, which 
would arise in the vertebrate from the mandibular, and not from 
the premandibular cavity. A similar muscle probably existed in 



© 





u 



mohlsup 



m obi tup 



, © 



mot>t sup 




Fig. 112. — A, Diagram op Position of Oblique Muscle in Scorpion ; B, Diagram 
of Transition Stage ; C, Diagram of Superior Oblique Muscle in Verte- 
brate. 

/.p., lateral eyes; c.e., central eyes; C.N., central nervous system; Al. t alimentary 
canal ; c, aqtieductus Sylvii. 

Eurypterus (M.obl. in Fig. 106, B), and, as in the case of the for- 
mation of the oculomotor group, derived from the recti group of the 
scorpion, would form the commencement of the superior oblique 
muscle in Thyestes and Tremataspis. 

It is instructive to notice that the original position of attachment of 
this muscle is naturally posterior to that of the oculomotor group of 
muscles, and that Furbringer, in his description of the eye-muscles 
of Petromyzon, asserts that this muscle in this primitive vertebrate 



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SEGMENTS OF TRIGEMINAL NERVE-GROUP 279 

form is not attached as in other vertebrates, but is posterior to the 
other muscles, so that he calls it the posterior rather than the superior 
oblique. The nature of the change by which the muscle known in 
the scorpion as the anterior dorso-plastron muscle (63) was probably 
converted into the superior oblique muscle of the vertebrate, is 
represented in the drawings Fig. 112, in which also are indicated 
the dwindling of the median eyes, and the progressive superiority of 
the lateral eyes, as well as the transformation of the recti muscle- 
group of the scorpion into the muscles supplied by the oculomotor 
nerve of the vertebrate. 

With respect to the external rectus muscle, it follows naturally 
that if the muscles (64) and (65) are to follow suit with the rest of 
the group and become attached to the lateral eyes, they must take 
up an external position. These two muscles, which always run 
together, as seen in Fig. 110, A, the one belonging to the prosoma 
and the other to the mesosoma, are represented by the mandibular 
muscle of Miss Piatt and the external rectus, the former derived 
from the walls of the last pro-otic head-cavity, the latter from the 
foremost of the opisthotic head-cavities. 

Such, then, is the simple explanation of the origin of the eye- 
muscles which f 0II0W8 from my theory, and we see that the successive 
alterations of the position of the orbit, and, therefore, of the globe of 
the eye with its muscles, as we pass from Thyestes to man, is the 
natural consequence of the growth of the frontal bone, i.e. of the brain. 

The Trigeminal Nerves and the Muscles supplied by them. 

Turning now to the evidence as to the number of ventral seg- 
ments, i.e. the motor and sensory supply to the prosomatic appendages 
afforded by the trigeminal nerve, we must, I think, come to the same 
conclusion as Dohrn, viz. that if there were originally seven dorsal or 
somatic segments in this region represented by : 1, Anterior cavity, 
muscle lost ; 2, 3, 4, 5, muscles of the premandibular cavity, sup. rectus, 
inf. rectus, int. rectus, inf. oblique, supplied by lllrd nerve; 6, 7, 
muscles of the mandibular cavity, sup. oblique, supplied by I Vth nerve 
and muscle lost, there must have been also seven corresponding 
ventral or splanchnic segments supplied by the trigeminal. At present 
the evidence for such segments is nothing like so strong as for the 
corresponding somatic ones; there are, however, certain suggestive 



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280 THE ORIGIN OF VERTEBRATES 

facts which point distinctly in this direction in connection with both 
the motor and sensory parts of the trigeminal. The origin of the 
trigeminal motor fibres in the central nervous system is most striking. 
We may take it for granted that a nucleus of cells giving origin to 
one or more segmental motor nerves will possess a greater or less 
longitudinal extension in the central nervous system, according to 
the number of fused separate segmental centres it represents. Thus 
a nucleus such as that of the IVth nerve or of the facial is small 
and compact in comparison to the extensive conjoint nucleus of 
the vagus and cranial accessory. 

Upon examination of the motor nucleus of the trigeminal, we 
find a compact or well-defined nucleus, the nucl. masticatorius, the 
nerves of which supply the masseter, temporal, and other muscles, 
so that the anatomical evidence at first sight appears to bear out 
van Wijhe's conclusion that the motor trigeminal supplies at most 
two segments. Further examination, however, shows that this is not 
all, for the extraordinary so-called descending root of the Vth must 
be taken into consideration in any question of the origin of the 
motor elements, just as the equally striking ascending root enters 
into the consideration of the meaning of the sensory elements of 
the Vth. 

It is not necessary here to discuss the controversy as to whether 
this descending root is motor or sensory. It is universally con- 
sidered at present to be motor, and is believed to supply, as 
Kolliker suggested, among other muscles, the m. tensor tympani and 
the m. tensor veli palati. It is thus described by Obersteiner — 

" From the region of the mid-brain the motor root receives an 
important addition of thick fibres, which form the cerebral or 
descending root. The large, round vesicular cells from which the 
fibres of the descending root arise form no single compact group, but 
are partly single, partly arranged like little bunches of grapes, as far 
as the region of the anterior corpora quadrigemina. The further we 
go brainwards, the smaller is the number of fibres. In the region 
of the anterior corpora quadrigemina, the few cells of origin are 
found more and more median; so that the uppermost trigeminal 
fibres descend in curves almost from the mid-line, as is shown by the 
exceptional occurrence of one or more of the characteristic cells above 
the aqueduct. At the height of the posterior commissure one finds 
the last of these trigeminal cells." 



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SEGMENTS OF TRIGEMINAL NERVE-GROUP 28 1 

The anatomy of the Vth nerve reveals, then, three most striking 
facts : — 

1. The motor nucleus of the Vth extends from the very commence- 
ment of the infra-infundibular region to nearly the commencement 
of the nucleus of the Vllth ; in other words, the motor nucleus of the 
Vth extends through the whole prosomatic region, just as it must 
have done originally if its motor nerves supplied the muscles of 
the prosomatic appendages. Such an extended range of origin is 
indicative of the remains of an equally extended series of segmental 
centres or ganglia. 

2. Of these centres the caudalmost have alone remained large and 
vigorous, constituting the nucleus masticatorius, which in the fish is 
divided into an anterior and posterior group, thus indicating a 
double rather than a single nucleus ; while the foremost ones have 
dwindled away until they are represented only by the cells of the 
descending root, the muscles of these segments being still represented 
by possibly the tensor veli palati and the other muscles innervated 
from these cells. 

3. The headmost of these cells takes up actually a position dorso- 
lateral to the central canal, so that the groups on each side nearly 
come together in the mid-dorsal line; a very unique and extra- 
ordinary position for a motor cell-group, but not improbable when we 
recall to mind Brauer's assertion as to the shifting of the foremost 
prosomatic ganglion-cells of the scorpion from the ventral to the 
dorsal side of the alimentary canal. 

On the sensory side the evidence is also suggestive, the question 
here being not so much the distribution of the sensory nerves as the 
number of ganglia belonging to each of the cranial nerves. 

With respect to this question, morphologists have come to the 
conclusion that there is a marked difference between spinal and 
cranial nerves, in that whereas the posterior root- ganglia of the 
spinal nerves arise from the central nervous system itself, i.e. from 
the neural crest, the ganglia of the cranial nerves arise partly from 
the neural crest, partly from the proliferation of cells on the surface 
of the animal ; and because of the situation of these proliferating 
epidermal patches over the gill-clefts in the case of the vagus and 
glossopharyngeal nerves, they have been called by Froriep and Beard 
branchial sense-organs. Beard divides the cranial ganglia into two 
sets, one connected with the neural ridges, called the neural ganglia, 



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282 THE ORIGIN OF VERTEBRATES 

and the other connected with the surface-cells, which he calls the 
lateral ganglia. This second set corresponds to Kupffer's epibranchial 
ganglia. Now it is clear that in the case of the vagus nerve, where, 
as is well shown in Ammocoetes, the nerve is not a single segmental 
nerve, but is in reality made up of a number of nerves going to 
separate branchial segments, the indication of such segments is not 
given by the main vagus ganglion or neural ganglion, but by the series 
of lateral ganglia. So also it is argued in the case of the trigeminal, 
that if in addition to the ganglion-cells arising from the neural crest 
separate ganglion-masses are found in the course of development, 
in connection with proliferating patches of the surface (plakodes, 
Kupffer calls them), then such isolated lateral ganglia are indications 
of separate segments, just as in the case of the vagus, even though 
the separate segments do not show themselves in the adult. So far 
the argument appears to me just, but the further conclusion that the 
presence of such plakodes shows the previous existence of branchial 
sense-organs, and, therefore, that such ganglia are epibrancJiial 
ganglia, indicating the position of a lost gill-slit, is not justified by 
the premises. If, as I suppose, the trigeminal nerve supplied a series 
of non-branchial appendages serially homologous with the branchial 
appendages supplied by the vagus, then it is highly probable that the 
trigeminal should behave with respect to its sensory ganglia similarly 
to the vagus nerve, without having anything to do with branchiae. 

Such plakodal ganglia, then, may give valuable indication of non- 
branchial segments as well as of branchial segments. The researches 
of Kupffer on the formation of the trigeminal ganglia in Ammocoetes 
are the chief attempt to find out from the side of the sensory ganglia 
the number of segments originally belonging to the trigeminal. The 
nature and result of these researches is described in my previous 
paper (Journal of Anatomy and Physiology, vol. xxxiv.), and it will 
suffice here to state that he himself concludes that the trigeminal 
originally supplied five at least, probably six, segments. As I have 
stated there, the evidence as given by him seems to me to indicate 
even as many as seven segments. 

In the full-grown Ammocoetes, as is well known, there are two 
distinct ganglia belonging to the trigeminal, the one the ganglion of 
the ramus ophthalmicus, the other the main ganglion. 

According to Kupffer the larval Ammocoetes possesses three sets 
of ganglia, not two, for between the foremost and hindmost ganglion 




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SEGMENTS OF TRIGEMINAL NERVE-GROUP 283 

he describes a nerve (x. t Fig. 113), with four epibranchial ganglia, 
which do not persist as separate ganglia, but either disappear or are 
absorbed into the two main ganglia (Fig. 113). This discovery of 
Kupffer's is very suggestive, for, as already stated, a transformation 
takes place when the Ammoccetea is 5 mm. long, so that the 
arrangement of the parts before that period is distinctly more 
indicative of the ancestral arrangement than any later one. 

If we use the name plakodal ganglia to represent that part of 
these ganglia which was originally connected with the skin, then 

V x V » IX * 



A HjulO'r II ve! 
Fig. 113. — Ganglia of the Cranial Nebves op an Ammoccetes, 4 mm. in length, 

PROJECTED ON TO THE MEDIAN PLANK. (After KUPFPER.) 

A-B, the line of epibranchial ganglia; au., auditory capsule; nc. t notochord; Hy. t 
tube of hypophysis ; Or. y oral cavity ; u.l. f upper lip ; l.l. lower lip ; vel., septum 
between oral and respiratory cavities ; V., VII., IX. y X., cranial nerves ; a?., 
nerve with four epibranchial ganglia. 

Kupffer's researches assert that in the larval Ammoccetes there were 
seven such plakodal ganglia, one in front belonging to the foremost 
trigeminal ganglion, two behind, parts of the hindmost ganglion, and 
four in between, which do not exist later as separate ganglia. 

In accordance with the views put forward in this book, a possible 
interpretation of these plakodal ganglia would be given as follows : — 

Beard, who, after Froriep, drew attention to this relation of the 
cranial ganglia to special skin-patches, has compared them with the 
parapodial ganglia of annelids, i.e. ganglia in connection with 
annelidan appendages ; whether we are here obtaining a glimpse of 
the far-off annelidan ancestiy of both arthropods and vertebrates it 
would be premature at present to say. It is natural enough to 
expect, on my view, to find evidence of annelidan ancestry in 



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284 THE ORIGIN OF VERTEBRATES 

vertebrate embryology (as has been so often asserted to be the case), 
seeing that undoubtedly the Arthropoda are an advanced stage of 
Annelida ; and, indeed, the way is not a long one when we consider 
Beecher's evidence that the Trilobita belong to the Phyllopoda, 
certainly a primitive crustacean group, which Bernard derives directly 
from the annelid group Chsetopoda. If, then, these plakodal ganglia 
indicate the former presence of appendages, we obtain this result : — 
The foremost ganglion on each side possesses one plakodal ganglion, 
and therefore indicates an anterior pair of appendages, possibly the 
chelicene. Then comes the peculiar nerve with four plakodal 
ganglia indicating on each side four appendages close together, 
possibly the endognaths. Then, finally,, on each side, the second 
large ganglion with two plakodal ganglia, indicating two pairs of 
appendages, possibly the ectognaths and the metastoma. 



Summary. 

The consideration of the history of the cranial segmentation shows that 
whereas, from the commencement of that history, the evidence for two ventral 
segments supplied by the trigeminal nerve is clear and unmistakable, later 
observers have tended more and more to increase the number of these segments, 
until at the present time the evidence is in favour of at least six, probably seven, 
as the number of segments supplied by the motor part of the trigeminal. 

So, also, the original evidence for the number of dorsal or somatic segments 
limits the number to three, innervated respectively by the oculomotor (III.), 
trochlear (IV.), and abducens (VI.) nerves, or rather two, since the last nerve 
belongs to the facial segment. The muscles which these three nerves supply 
are derived respectively from the walls of the prentandibular, mandibular, and 
hyoid coelomic cavities. 

Later evidence points strongly to the conclusion that the oculomotor nerve 
and the premandibular cavity represent not one segment but the fusion of 
four, while the mandibular cavity represents two segments. In addition to 
these, Miss Piatt has discovered a still more anterior head-cavity, which she has 
named the anterior cavity, so that the pro-otic segments on this reckoning are 
seven in number, viz. : (1) the anterior cavity. (2, 3, 4, 5) the premandibular cavity, 
(6, 7) the mandibular cavity. The somatic muscles belonging to these dorsal 
segments are the eye-muscles, which are all dorso-ventral in position, and are 
not the same as the longitudinal somatic muscles, but belong to a distinct dorso- 
ventral segmental group, the only representative of which at present known in 
the mesosomatic region is the external rectus innervated by the Vlth nerve. 

These head-cavities, and these muscles of the vertebrate, resemble the 
corresponding cavities and muscles of the invertebrate to an extraordinary 



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SEGMENTS OF TRIGEMINAL NERVE-GROUP 285 

degree, so that it becomes easy to see how the dorso-ventral muscles of the 
prosomatic segments of the latter have become converted into the eye-muscu- 
lature of the former. The most powerful proof of all that such a conversion 
has taken place is that a natural and simple explanation is at once given of 
the extraordinary course taken by the IVth or trochlear nerve. Ever since 
neurology began, the course of this nerve has arrested the attention of anato- 
mists. Why should just this one pair of nerve-roots of all those in the whole 
body be directed dorsal wards instead of ventralwards, and cross each other in the 
valve of Vieussens, each to supply a simple eye-muscle (the superior oblique) 
belonging to the other side ? For generations anatomists have wondered and 
found no solution, and yet, without any straining of hypotheses, in consequence 
simply of the investigation of the anatomy of the corresponding pair of muscles 
in the scorpion group, the solution is immediately apparent. 

This pair of muscles alone, of all the musculature attached to the carapace, 
crosses the mid-dorsal line to be attached to the other side, thus carrying its 
nerve with it to the other side; by a continuation of the same process the 
relation of the trochlear to the superior oblique muscle can be explained. 

The comparison of the eye-muscles of the vertebrate with the dorso-ventral 
segmented muscles of the invertebrate makes the number and nature of the 
pro-otic segments much clearer. 



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CHAPTER IX 
THE PRO SO MA TIC SEGMENTS OF AMMOCCETES 

The prosomatic region in Ammocoetes.— The suctorial apparatus of the adult 
Petromyzon. — Its origin in Ammocoetes. — Its derivation from appendages. 
— The segment of the lower lip or metastomal segment. — The tentacular 
segments. — The tubular muscles. — Their segmental arrangement. — Their 
peculiar innervation. — Their correspondence with the system of veno- 
pericardial muscles in Limulus. — The old mouth or palaeostoma.— The 
pituitary gland. — Its comparison with the coxal gland of Limulus. — 
Summary. 

In the last chapter it was seen not to be incompatible with both the 
anatomical and morphological evidence to look upon the trigeminal 
nerves as having originally supplied the seven prosomatic pairs of 
appendages of the invertebrate ancestor, the foremost of which, the 
chelicerse, and the four pairs of endognaths dwindled away and became 
insignificant, leaving as trace of their former presence the descending 
root of the Vth nerve ; while the two hindmost pairs, the ectognaths 
and the chilaria, or metastoma, remained vigorous and developed, 
leaving as proof of their presence the nucleus masticatorius. Evi- 
dence in favour of this suggestion and of the nature of the dwindling 
process is afforded when we examine what the trigeminus does supply 
in Ammocoetes. In all vertebrates this nerve supplies the great 
muscles of mastication which, in all gnathostomatous fishes, move 
the jaws. The lowest fishes, the cyclostomes, possess no jaws ; they 
take in their food by attaching themselves to their prey and by 
means of rasping teeth situated in serried rows within the circular 
mouth, combined with a powerful suctorial apparatus, they suck the 
juices of the fish they feed upon. Not possessing jaws, they feed 
by suction on the living animal, a method of feeding which gives 
them no more claim to be classed as parasitic animals than the 
whole group of spiders which feed in a similar manner on living 
flies. 



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THE PROSOMATIC SEGMENTS OF AMMOCiETES 287 

The Origin of the Suctorial Apparatus of Petromyzon. 

This powerful suctorial apparatus is innervated entirely by the 
trigeminal nerve, so that here in its muscular arrangements any 
original segmental arrangement of the muscles of mastication might 
be expected to be visible. It consists of a large rod or piston, to 
which are attached powerful longitudinal muscles; a large muscle, 
the basilar muscle, which assists the piston in producing a vacuum, 
and annular muscles around the circular lip. 

Turn now to the full-grown larval form, Ammoccetes, an animal 
in the case of Pciromyzon Plancri as large as the full-grown Petro- 
myzon, and seek for this musculature. There is, apparently, no sign 
of it, no suctorial apparatus whatever, only, as already mentioned, 
an oral chamber bounded by the lower and upper lips and the 
remains of the septum between it and the respiratory chamber — the 
velar folds. Attached to its walls a number of tentacles are situated, 
which form a fringe around and within the mouth. Most extra- 
ordinary is the contrast here between the larval and the adult 
stages ; in the former, no sign of the suctorial apparatus, but simply 
tentacles and velar folds ; in the latter, no sign of tentacles or of 
velar folds, but a massive suctorial apparatus. 

In order, then, to understand the origin of the muscles of masti- 
cation, it is necessary to study the changes which occur at trans- 
formation, and thus to find out how the suctorial apparatus of the 
adult arises. This most important investigation has been under- 
taken by Miss Alcock, and owing to the kindness of Mr. Millington, 
of Thetford, we have been able to obtain a better series in the trans- 
formation process than has ever been obtained before. Miss Alcock 
has not yet published her researches, but has allowed me to make 
use of some of her facts. 

An enormous proliferation of muscular tissue takes place with 
great rapidity during this transformation, which causes the disappear- 
ance of the tentacles, and gives origin to the suctorial apparatus. 
The starting point of this proliferation can be traced back in all 
cases to little groups of embryonic tissue found below the epithelial 
lining of the oral chamber in Ammocnetes. Of these groups the most 
conspicuous one is situated at the base of the large median ventral 
tentacles. Others are situated at the base of the tentacular ridge. 
Further, although this extraordinary change takes place in the 



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288 



THE ORIGIN OF VERTEBRATES 



peripheral organ, no marked difference occurs in the arrangement 
of the nerves issuing from the trigeminal motor centre, no new 
nerves are formed to supply the new muscles, but every motor nerve- 
fibre and the motor cell from which it arises increases enormously in 
size, and these giant nerve-fibres thus formed split into innumerable 
filaments corresponding with the proliferation of the muscular elements. 
The clue, then, to the origin of the suctorial apparatus and of the 
nature of the original organs supplied by the trigeminal is afforded 
in this case, as in all other similar inquiries, by the central nervous 
system and its outgoing nerves. Here is always the citadel, the 
fixed seat of government, here is 'headquarters/ from which the 
answers to all our inquiries must originate. 

The Trigeminal Nerve of Ammoixetks. 

Striking is the answer. In Fig. 114, Miss Alcock has drawn the 
distribution of the trigeminal nerve as traced by her through a series 




Fig. 114.— Distribution of Trigeminal Nerve in Ammocketes. 

ps. bi'.y pseudo-branchial groove; met., nerve to lower lip, or metastomal nerve; /., 
nerve to tongue ; knt. y nerve to tentacles. The mandibular and internal maxil- 
lary nerves are coloured red ; the purely sensory nerves to the external surface 
are coloured black. 

of sections. It arises, as is well known, from two separate ganglia, of 
which the foremost gives rise to a purely cutaneous nerve, the oph- 
thalmic nerve, and the hindmost to three nerves, the most posterior 
of which is purely cutaneous and passes tailwards over the ventral 
branchial region, as shown in the figure ; the other two nerves, both 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 289 

of which contain motor fibres, are called by Hatschek the mandibular 
and maxillary nerves. Of these the mandibular or velar nerve {met) 
is a large, conspicuous nerve, which arises so separately from the 
rest of the trigeminal as almost to deserve the title of a separate 
nerve. When it leaves the large posterior ganglion, it passes into 
the anterior part of the velum, runs along with the tubular muscles, 
which it supplies, to the ventral surface as far as the junction of the 
lower lip with the thyroid plate, and has not been followed further by 
Hatschek. Miss Alcock, however, by means of serial sections, has 
traced it further, and shown that at this point it turns abruptly 
headwards to terminate in the muscles of the lower lip. If, then, 
as suggested, the lower lip represents the metastoma — the last pair 
of prosomatic appendages — then this mandibular or velar nerve 
represents that segmental nerve. 

The other nerve — the maxillary nerve of Hatschek — which con- 
stitutes the larger part of the trigeminal, passes forwards from the 
ganglion, and at a point somewhere about the anterior region of the 
eyeball, divides into two, an external (black in Fig. 114) and an 
internal (red in Fig. 114) nerve. The external branch is apparently 
entirely sensory, and supplies the external surfaces of the upper and 
lower lips. The internal branch is mainly motor, and supplies the 
muscles of the upper lip ; it contains also the nerves of the tentacles. 

The nerve to the median ventral tentacle (t.) or tongue leaves the 
internal division of the maxillary immediately after its separation 
from the external ; it runs ventralwards, and at the same time passes 
internally until it reaches a position between the muco-cartilage and 
the epithelium lining the cavity of the throat. It then turns, and 
passing posteriorly (towards the tail) to the point where the median 
ventral tentacle is attached to the lower lip, it supplies some very 
rudimentary-looking muscles which run from the tentacle to the 
adjoining surface, and no doubt serve to move the tentacle from side 
to side. A portion of the nerve still continues to run along the side 
of the median ventral ridge, as far back as the point where the 
muscles of the hyoid segment pass round to the ventral side between 
the velum and the thyroid; in fact, this small nerve passes along 
the whole length of the median ventral ridge. 

This description shows that the trigeminal nerve divides itself 
into two groups : the one represented black in the figure, which is 
purely cutaneous and sensory, corresponding, in the main, according 

u 



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2 go THE ORIGIN OF VERTEBRATES 

to my theory, to the epimeral nerves of Limulus ; the other coloured 
red, which supplies muscles belonging to the visceral or splanchnic 
muscle-group, and contains also the nerves to the tentacles. 

This latter group, which is formed by two distinct well-defined 
nerves, viz. the mandibular and the internal branch of the maxillary, 
corresponds, according to my theory, to the amalgamated nerves of 
the prosomatic appendages, and is clearly divisible into three distinct 
nerves — 

1. The lower lip-nerve or the metastomal nerve (met.). 

2. The tongue-nerve (£.). 

3. The nerve (tent.) to the upper lip and tentacles. 

Of these three pairs of nerves it is suggested that the first pair 
were derived from the nerves to the metastomal appendage. The 
second pair of nerves ought, on this theory, originally to have sup- 
plied the pair of appendages immediately in front of the metastoma 
— that is, the pair of ectognaths, and therefore the ventral pair of 
tentacles, known as the tongue, would represent the last remnant of 
these ectognaths. Similarly, the other tentacles would represent the 
endognaths, and therefore the third pair of nerves would represent 
the fused nerves to these concentrated endognaths, which, in the 
Eurypterids, stand aloof from the ectognaths. 

Let us consider these three propositions separately. In the first 
place, have we any right to attribute segmental value to the man- 
dibular nerve ? What evidence is there of segments in this region 
in Ammocoetes ? 

The Segment of the Lower Lip, or Metastomal Segment. 

We have seen that in the branchial or mesosomatic region the 
segments corresponding to the mesosomatic appendages were mapped 
out by means of their supporting or skeletal structures, their seg- 
mental muscles, and their nervous arrangements, as well as by the 
arrangement of the branchiae. Similarly, the segments in front of 
the branchial region, corresponding to the prosomatic appendages, 
ought to be definable by the same means, although, owing to the 
absence of branchiae and the greater concentration in this region, 
the separate segments would probably not be so conspicuous. 

The last segment considered was the segment belonging to the 
Vllth nerve corresponding to the opercular appendages of the 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 29 1 

Eurypterid. The segment immediately in front of this is the next 
for consideration, viz. that corresponding to the chilarial appendages 
or metastoma ; and as the basal part of this pair of appendages was 
fused with the basal part of the operculum, the one cannot be dis- 
cussed without the other ; therefore, the segment to which the lower 
lip belongs must be considered in connection with and not apart 
from the thyro-hyoid segments already dealt with. 

In Chapter V., p. 188, I stated that the supporting bars of the 
foremost mesosomatic segments, the thyro-hyoid segments, differed 
from the cartilaginous bars of the branchial segments, in that they 
were composed of muco-cartilage. Also in addition to the muco- 
cartilaginous skeletal bars, a ventral plate of muco-cartilage exists in 
Ammoccetes which covers over the thyroid gland. 

Similarly in the prosomatic segments the skeletal bars are com- 
posed of muco-cartilage and the ventral plate of muco-cartilage 
continues forward as the plate of the lower lip. ' It is of special 
interest, in connection with the segments indicated by such support- 
ing structures, to find that this special tissue is entirely confined to 
the head-region, and disappears absolutely at transformation, thus indi- 
cating the ancestral nature of the segments marked out by its presence. 

This muco-cartilaginous skeleton is the key to the whole position, 
and requires, therefore, to be understood. It is of great importance, 
not only because it demonstrates the position of the segments in 
Ammoccetes which characterized its invertebrate ancestor, but also 
because it possesses a structure remarkably similar to that found 
in the head-plates of the most ancient fishes. For the present I will 
confine myself to the consideration of this muco-cartilaginous skeleton 
as evidence of the relationship of Ammoccetes to the Eurypterids, 
and in the next chapter will show how absolutely the same skeleton 
corresponds to that of the Cephalaspidae, so that Ammoccetes is 
really a slightly modified Cephalaspid, the larval form of which was 
Eurypterid in character. 

In Chapter IV., Figs. 63, 64, I have given a representation of the 
ventral and dorsal views of an Ammoccetes cut in half horizontally. 
Such a section shows with great clearness the series of branchial 
appendages with their segmental muscles and cartilaginous bars 
which form the branchial segments innervated by the IXth and Xth 
nerves, according to my view of the branchial unit. As is seen (Fig. 
64 or 115), the skeletal bar of the hyoid or opercular appendage, 



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Tr. -^ 



Set.- 



Fig. 115. — Dons a i. 

HALF OF HeAO- 
REGIOX OF AM- 
MOCCETKM. 




Pxt. 



-Inf. 



7V., trabecule ; 
rit. % pituitary 
ppace; Inf., iii- 
d i b u 1 u m ; 
median ser- 
r;it» ,1 flange of 
vclnr folds. 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 293 

which is clearly serially homologous with the other branchial bars, is 
composed of muco-cartilage, and not of cartilage. If we follow this 
series of horizontal sections nearer to the origin of the cartilaginous 




Fia. 116. — Horizontal Section through the Anterior Part of Ammoccetes, 

IMMEDIATELY VENTRALLY TO THE AUDITORY CAPSULE. 

sk x -sli $ , skeletal bars; w r w 4 , striated visceral muscles; mt l - j mt i1 tubular muscles; 
br x -br ly branchiae; ir., trabecule; inf., inf undibulum ; ped., pedicle; V., tri- 
geminal nerve. Muco-cartilage, red ; soft cartilage, blue ;' hard cartUage, purple. 

bars from the sub-chordal cartilaginous rod on each side of the noto- 
chord, we obtain a picture, as in Fig. 116, in which each branchial 
segment is defined by the section of the branchial cartilaginous bar 



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294 



THE ORIGIN OF VERTEBRATES 



{sk^ 8k 5 ), by the section of the separate branchiae (&r 2 , &r 3 ), and by the 
separate segmental muscles arranged round each bar, these muscles 
being partly ordinary striated (m*, m 5 ), partly tubular (mt 3 , mt 4 ). The 
uppermost of these branchial segments shows the same arrangement ; 
(ska) is the branchial skeletal bar, which is now composed of muco- 
cartilage, not cartilage ; (br x ) is the branchiae in the same situation as 
the others, but here composed of glaudular rather than of respiratory 
epithelium, while the ordinary striated branchial muscles of this seg- 
ment are marked as (m 9 ), being separated from the tubular muscles of 
the segment (mtf 2 ), owing to the large size of the blood-space in which 




Fig. 117. — Sagittal Lateral Section through the Anterior Part op Ammoc<etes. 
Lettering and colouring same as in Fig. 116. and., auditory capsule ; j.v., jugular vein. 

these latter muscles are lying. In front of this segment so defined 
we see again another well-marked skeletal bar (sk?) of muco- cartilage, 
evidently indicating a similar segment anterior to the hyoid segment. 
In connection with this bar there are no branchioe, but again we see 
two sets of visceral muscles, the one ordinary striated, marked (m*), 
and the other tubular, marked (mti). Here, then, the section indicates 
the existence of a segment of the same character as the posteriorly 
situated branchial segments but belonging to a non-branchial region 
— a segment which would represent a non-branchial appendage, the 
last, therefore, of the prosomatic appendages. Let us, then, follow 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 295 

out these two segmental muco-cartilaginous bars and their attendant 
muscles, and see to what sort of segments their investigation 
leads. 

The bar which comes first for consideration (sks) arises imme- 
diately behind the auditory capsule from the first branchial cartilage 
very soon after it leaves the sub-chordal cartilaginous ligament ; the 
soft cartilage of the sub-chordal ligament ceases abruptly in its 
extension along the notochord at the place where the hard cartilage 
of the parachordal joins it, and in a sense it may be said to leave the 
notochord at this place and pass into the basal part of the first branchial 
bar. The most anterior continuation of this branchial system is this 
muco cartilaginous bar (s& 3 ), which passes forward and ventral wards, 
being separated from the axial line by the auditory capsule (cf. Fig. 
118, A, B, C). Its position is well seen in a sagittal section, such 
as Fig. 117. It follows absolutely the line of the pseudo-branchial 
groove (})s. 6r., Fig. 114), and ventrally joins the plate of muco- 
cartilage which covers the thyroid gland. It forms a thickened 
border to this plate anteriorly, just as the branchial cartilaginous 
bars border it posteriorly. In fact, it behaves with respect to the 
hyoid segment in a manner similar to the rest of the cartilaginous 
bars with respect to their respective segments. 

It represents, although composed of muco-cartilage, the cartila- 
ginous bar of the operculum in Limulus, which also forms the termi- 
nation of the branchial cartilaginous system, as fully explained in 
Chapter III. ; it may therefore be called the opercular bar. 

The next bar (sk 2 ) is extremely interesting, as we are now out of 
the branchial or mesosomatic region, and into the region corresponding 
to the prosoma. It starts from a cartilaginous projection made of 
hard cartilage, just in front of the auditory capsule, called by Farker 
the 'pedicle of the pterygoid' — a projection (ped.) which defines the 
posterior limit of the trabecule on each side, where they join on to 
the parachordals, — and winding round and bolow the auditory capsule, 
joins the opercular bar (cf. Fig. 118), to pass thence into and form part 
of the muco-cartilaginous plate of the lower lip. In the section figured 
(Fig. 116), this projection of hard cartilage is not directly continuous 
with (sA- 2 ), owing to a slight curvature in the bar; the next few 
sections show clearly the connection between (iwd.) and (sic*), and 
consequently the complete separation by means of this bar of the 
hyoid segment from the segment in front. In the figures, the hard 



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296 



THE ORIGIN OF VERTEBRATES 

s^ 2 







Fig. 118.— Skeleton of Heap-Region of Ammoccetes. A, Lateral View; B, 
\ 1 MiaL View; C, Dorsal View. 

Muco-cartilage, red ; soft cartilage, blue ; hard cartilage, purple. sk u sk t , sk 3 , 
skeletal bars ; c.c. t position of pineal eye ; na. cart., nasal cartilage ; pcd., pedicle ; 
cr. t cranium ; fie, notochord. 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 297 

cartilage is coloured purple, the soft cartilage blue, and the muco- 
cartilage red, so that the position of this bar is well shown. This 
bar may be looked upon as bearing the same relation to the muco- 
cartilaginous plate of the lower lip as the opercular bar does to the 
muco-cartilaginous plate over the thyroid ; and seeing that these two 
plates form one continuous ventral head-shield of muco-cartilage 
(Fig. 118, B), and also that this bar fuses with the opercular bar, we 
may conclude that the segment represented by the lower lip is 
closely connected with the hyoid or opercular segments. In other 
words, if the lower lip arose from the metastoma, then this pair of 
skeletal bars might be called the metastomal bars, which formed the 
supporting skeleton of the last pair of prosomatic appendages and, as 
is likely enough, arose in connection with the posterior lateral horns 
of the plastron; these posterior lateral horns, like the rest of the 
plastron, would give rise to hard cartilage, and so form in Ammoccetes 
the two lateral so-called pterygoid projections. 

In the branchial region the muscles which marked out each 
branchial segment were of two kinds— ordinary striated visceral 
muscles and tubular muscles. Of these the former represented the 
dorso-ventral muscles of the branchial appendages, while the latter 
formed a separate group of dorso-ventral muscles with a separate 
innervation which may have been originally the segmental veno- 
pericardial muscles so characteristic of Limulus and the scorpions. 
In Figs. 116, 117, the grouping of these muscles in each branchial 
segment is well shown, and it is immediately seen that the hyoid 
segment possesses its group of striated visceral muscles (m 8 ) supplied 
by the Vllth nerve in the same manner as the posterior groups, as 
has already been pointed out by Miss Alcock in her previous 
paper. Passing to the segment in front, Fig. 116 shows that the 
group of visceral muscles (m%) corresponds in relative position with 
respect to the metastomal bar to the hyoid muscles with respect 
to the opercular bar or to the branchial visceral muscles with 
respect to each branchial bar. What, then, is this muscular group ? 
The series of sections show that these are the dorso-ventral muscles 
belonging to the lower lip, which, as seen in Fig. 119 (M.), form a 
well-marked muscular sheet, whose fibres interlace across the mid- 
ventral line of the lower lip. This group of lower lip-muscles is very 
suggestive, for these muscles arise, not from the trabecule, but from 
the front dorsal region of the cranium, just in front of the two lateral 



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298 



THE ORIGIN OF VERTEBRATES 



eyes. In Fig. 117 the dorsal part is seen cut across on its way to its 
dorsal attachment. Such an origin is reminiscent of the tergo-coxal 
group of muscles, arising, as they do, from the primordial cranium 
and the tergal carapace, and suggests at once that when the chilarial 

appendages expanded to form a meta- 
stoma, their tergo-coxal muscles formed 
a sheet of muscles similar to those of 
the lower lip of Ammoccetes, by which 
the movements of the metastoma were 
effected. The posterior limit of these 
muscles ventrally marks out the junction 
of the segment of the lower lip with 
that of the thyroid; in other words, 
indicates where the metastoma had fused 
ventrally with the operculum (Fig. 117). 
Besides the striated visceral muscles, 
each branchial segment possesses its own 
tubular muscles, shown in Fig. 116 (mt 9 ) 
and (mt A ). As the section shows, there 
is clearly a group of tubular muscle- 
fibres belonging to the hyoid segment 
(mt 2 ), and also another group belonging 
to the segment in front of the hyoid 
{mti)\ so that, judging from this section, 
each of these segments possesses its 
own tubular musculature just as do the 
branchial segments, the difference being 
that the tubular muscles are more 
separated from the striated visceral 
group than in the true branchial seg- 
ments, owing to the size of the blood- 
spaces surrounding them. What, then, 
are these two groups of muscles? 
Tracing thein in the series of sections, 
both groups are seen to belong to the system of velar muscles, 
forming an anterior and a posterior group respectively ; and we see, 
further, that there is not the slightest trace of any tubular muscles 
anterior to these muscles of the velum. 

In the living Ammoccetes the velar folds on each side can be seen 




Fig. 119. — Ventral View of 
Head-Region of Ammocxetes. 

Th. t thyroid gland; Jf., lower 
lip, with its muscles. 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 299 

to move synchronously with the movements of respiration, con- 
tracting at each expiration, and thus closing the slit by which the 
oral and respiratory chambers communicate, and so forcing the 
waters of respiration through the gill-slits, as described by Schneider. 
Such a fact is clear evidence that these tubular muscles of the velar 
folds belong to the same series as the tubular muscles of the branchial 
segments, so that if, as I have already suggested, the latter muscles 
were originally the veno-pericardial muscles of segments corre- 
sponding to the branchial appendages, then the former would represent 
the veno-pericardial muscles of the segments corresponding to the 
opercular and metastomal appendages. What, then, are these velar 
folds, and how is it that the tubular muscles of these two segments 
become the velar muscles ? I will consider, in the first instance, the 
posterior group of muscles (mt^) in Fig. 116. 

It has already been pointed out that the tubular muscles of the 
branchial segments are dorso-ventral, but do not run with the 
ordinary constrictors, having separate attachments and running part 
of their course internally to and part externally to the ordinary con- 
strictors. At first sight, as is usually stated, the hyoid segment does 
not appear to possess tubular muscles at all. If, however, we follow 
the posterior group of velar muscles (mt 2 ), we see (Fig, 117) that 
they pass between the auditory capsule and the opercular bar (ska) of 
muco-cartilage to reach the region of the jugular vein (J.v.) posteriorly 
to the auditory capsule, so that their dorsal origin bears the same 
relation to the hyoid segment as the dorsal attachment of the rest of 
the tubular muscles to their respective segments. Further, these 
muscles run along the length of the velar fold, and are attached 
ventrally on each side of the thyroid gland, so that their ventral 
attachment also corresponds in position, as regards the hyoid segment, 
with the ventral attachment of the rest of the tubular muscles as regards 
their respective segments. 

This ventral attachment is shown in Fig. 119 on each side of the 
thyroid, and in Fig. 120 (mt 2 ) ; while in Fig. 117 the fibres are seen 
converging to this ventral position. In other words, this large 
posterior muscle of the velar folds is a dorso-ventral muscle, and 
would actually take the same position in the hyoid segment as the 
dorso-ventral tubular muscles in the other branchial segments, if 
the velum were put back into its original position as the septum ter- 
minating the branchial chamber. Conversely, the presence of these 



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300 THE ORIGIN OF VERTEBRATES 

hyoid tubular muscles in the velum gives evidence that the oper- 
cular segment takes part in the formation of the septum, as already 
suggested. 

Miss Alcock, in her paper, speaks of tubular muscles belonging 
to the hyoid segment, which are attached to the muco-cartilage. 
Schaffer-also speaks of certain tubular muscles belonging to the velar 
group as piercing the muco-cartilage (h. r. s.) in his figures 24 and 25, 
i.e. the metastomal bar, near its junction with the opercular bar. In 
my specimens there is a distinct group of tubular muscles which 
pierce the opercular bar of muco-cartilage at its junction with the 
metastomal bar, and pass into the posterior group of velar muscles. 
They clearly belong to the hyoid segment, as Miss Alcock supposed, 
but are not attached to the muco-cartilage. It is possible that they 
represent a different group to those already considered, and suggest 
the possibility that this opercular or thyro-hyoid segment is double 
with respect to its original veno-pericardial muscles as well as in 
other respects. 

The anterior group of tubular muscles (nU\, Figs. 116, 117) 
belonging to the same segment as the metastomal bar must now be 
taken into consideration. Very different is their origin to that of 
the posterior group : they arise close up against the eye, and have 
given rise to Kupffer's and Hatschek's misconception that the superior 
oblique muscle of the eye arises from a part of the velar muscu- 
lature. Naturally, as Neal has pointed out, they have nothing to do 
with the eye-muscles ; the superior oblique muscle is plainly in its 
true place entirely apart from these velar muscles, which form the 
foremost group of the segmental tubular muscles. They pass into 
the anterior part of the velar folds and run round to the ventral 
side just in the same way as does the posterior group. This anterior 
group of tubular muscles represents the veno-pericardial muscle of 
the segment immediately in front of the opercular, i.e. the metasto- 
mal segment, and is the foremost of these veno-pericardial muscles. 
Its presence shows that the velar folds, formed as they were by the 
breaking down of the septum, are in reality part of two segments, 
viz. the opercular and the metastomal, which have fused together 
in their basal parts, and by such fusion have caused the inter-relation- 
ship between the Vllth and Vth nerves, so apparent in the anatomy 
of the vertebrate cranial nerves. 

A further piece of evidence that this anterior portion of the velum 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 301 

.belongs to the same segment as the lower lip is the fact that in 
addition to the tubular muscles a single ordinary striated muscle is 
found in the velum which, like the muscles of the lower lip, is 
innervated by this same mandibular nerve. 

This muscle is attached laterally to the muco-cartilage of the 
metastomal bar (sk^) at its junction with the muco-cartilage of the 
lower lip, and spreads out into a number of strands which are 
attached at intervals along the whole length of the free anterior 
edge of the velum. It is the only non-tubular muscle belonging 
to the velum, and by its contraction it draws the anterior portions 
of the velar folds apart from each other, and so opens the slit 
between them, through which the food and mud must pass. Clearly 
from its position it does not belong to the original tergo-coxal group 
of muscles as do those of the lower lip ; it must have been one of the 
intrinsic muscles of the metastoma itself. 

This anterior portion of the velar folds affords yet another 
striking hint of the correctness of my comparison of the lower lip 
segment of Ammocoetes with the chilaria of Limulus or the metas- 
toma of Eurypterus ; for the most dorsal anterior portion, which at 
its attachment possesses a wedge of muco-cartilage, forms a separate, 
well-defined, rounded basal projection marked Ser.in Fig. 115, and B 
in the accompanying Fig. 120. This is that part of the velar folds 
which comes together in the middle line and closes the entrance into 
the respiratory chamber. The epithelial surface here is most striking 
and suggestive, for it is markedly serrated, being covered with a 
large number of closely-set projections or serrae. The serration of 
the surface here is of so marked a character that Langerhans con- 
sidered this part of the velar folds to act as a masticating organ, 
grinding and rasping the food and mud which passed through the 
narrow slit. In fact, Langerhans supposed that this portion of the 
velum acted in a manner closely resembling the action of the gnatho- 
bases of the prosomatic appendages in Limulus or the Eurypteridae. 

This suggestion of Langerhans is surely most significant, con- 
sidering that this somewhat separate portion of the velum, to which 
he assigns such a function, is in the very place where the gnathite 
portion of the metastomal appendages would have been situated if it 
were true that the lower lip and anterior portion of the velum of 
Ammoccetes were derived from the metastoma. 

In addition to this marked serrated edge the whole surface of 



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302 



THE ORIGIN OF VERTEBRATES 



the anterior portion of the velum is covered over with a scale-like 
or tubercular pattern remarkably like the surface-ornamentation 




^.ta.-L. 



Fig. 120.— Ammoc<etes cut open in Mid-Ventral Line to show Position op 
Velum; Velab Folds removed on one side. 

tr. t trabecular; vcl., velum; D. t anterior gnathic portion of velum; ps. br., pseudo- 
branchial groove ; m tt muscles of lower lip segment ; m„ muscles of thyTO-hyoid 
segment ; mt t , insertion of tubular muscles of velum near thyroid. 

seen in many of the members of the ancient group EuryptericUe. In 
Fig. 121 I give a picture of this surface-marking of the velum. It 
is striking to see that just as in the case of the invertebrate this 
marking and these seme are formed simply by the cuticular surface 
of the epithelial cells ; a surface which, according to Wolff, possibly 
contains chitin. The interpretation which I 
would give of the velar folds is therefore 
as follows : — 

They represent the fused basal parts of 
the opercular and metastomal appendages, 
the gnatho-bases of the latter still retaining 
in a reduced degree their rasping surfaces, 
because, owing to their position on each side 
of the opening into the respiratory chamber 
they were still able to manipulate the food as it passed by them 
after the closure of the old mouth. 

The whole evidence points irresistibly to the conclusion that the 
mandibular or velar nerve of the trigeminal does supply a splanchnic 




Fig. 121.— Surface View 
of Anterior Surface 
of Velum. 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 303 

segment which is, in all respects, comparable with the segments 
supplied by the facial, glossopharyngeal, and vagus nerves, except 
that it does not possess branchiae. This simply means that the 
appendages which these nerves originally supplied were prosomatic, 
not mesosomatic, and corresponded, therefore, to the chilarial or 
metastomal appendages. 

A comparison of the ventral surface of Slimonia, as given in 
Fig. 8, p. 27, with that of Ammocoetes (Fig. 119), when the thyroid 
gland and lower lip muscles have been exposed to view, enables the 
reader to recognize at a glance the correctness of this conclusion. 

The Tentacular Segments and the Upper Lip. 

Anterior to this metastomal segment, Fig. 116 shows a group of 
visceral muscles, mi, and yet again a muco-cartilaginous bar, ski, but, 
as already stated, no tubular muscles. These visceral muscles indicate 
the presence in front of the lower lip-segment of one or more segments 
of the nature of appendages. The muscles in question (mi) are the 
muscles of the upper lip, the skeletal elements form a pair of large 
bars of muco-cartilage (ski), which start from the termination of the 
trabecule, and pass ventral wards to fuse with the muco-cartilaginous 
plate of the lower lip (Figs. 117 and 118). This large bar forms the 
tentacular ridge on each side, and gives small projections of muco- 
cartilage into each tentacle. In addition to this tentacular bar, a 
special bar of muco-cartilage exists for the fused pair of median 
tentacles, the so-called tongue, which extends in the middle line 
along the whole length of the lower lip, being separated from the 
muco-cartilaginous plate of the lower lip by the muscles of the lower 
lip. This tongue bar of muco-cartilage joins with the muco-cartilage 
of the lower lip at its junction with the thyroid plate, and also with 
the tentacular bar just before the latter joins the muco-cartilaginous 
plate of the lower lip. This arrangement of the skeletal tissue 
suggests that the pair of tentacles known as the tongue stand in a 
category apart from the rest of the tentacles ; a suggestion which is 
strongly confirmed by the separate character of its nerve-supply, as 
already mentioned. 

For three reasons, viz. the separateness both of their nerve-supply 
and of their skeletal tissue, and the importance they assume at trans- 
formation, this pair of ventral tentacles must, it seems to me, l>e put 



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304 THE ORIGIN OF VERTEBRATES 

into a separate category from the rest of the tentacles. On the other 
hand, the innervation of the rest of the tentacles by a single nerve 
which sends off a branch as it passes each one, together with the 
concentration of their skeletal elements into a single bar, with pro- 
jections into each tentacle, points directly to the conclusion that these 
tentacles must be considered as a group, and not singly. 

I suggest that these tentacles are the remains of the ectognaths 
and endognaths; the tongue representing the two ectognaths, and 
the four tentacles on each side the four pairs of endognaths. 

As we see, this method of interpretation attributes segmental 
value to the tentacles, a conclusion which is opposed to the general 
opinion of morphologists, who regard them as having no special 
morphological importance, and certainly no segmental value. On 
the other hand, the importance of the pair of ventral tentacles, the 
' tongue ' of Rathke, which lie in the mid-line of the lower lip, has 
been shown by Kaensche, Bujor, and others, all of whom are 
unanimous in asserting that at transformation they are converted 
into that large and important organ the piston or tongue of the adult 
Petromyzon. It is supposed that the rest of the tentacles vanish 
at transformation, being absorbed ; they appear to me rather to take 
part in the formation of the sucking-disc, so that I am strongly 
inclined to believe that the whole of the remarkable suctorial 
apparatus of Petromyzon is derived from the tentacles of Ammoccetes. 
In other words, on my view, a conversion of the prosomatic appen- 
dages into a suctorial apparatus takes place at transformation, just 
as is frequently the case among the Arthropoda. 

It is to the arrangement of the muscles that we look for evidence 
of segmental value. As long as it was possible to look upon these 
tentacles as mere sensory feelers round the mouth entrance, it was 
natural to deny segmental value to them. Matters are now, how- 
ever, totally different since Miss Alcock's discovery of the rudimen- 
tary muscles at the base of the tentacles and their development at 
transformation. If these muscles represent some of the appendage 
muscles belonging to the foremost prosomatic segments just as the 
ocular muscles represent the dorso-ventral somatic muscles of those 
same segments, then we may expect ultimately to be able to give 
as good evidence of segmentation in their case as I have been able 
to give in the case of these latter muscles ; for the two sets of muscles 
are curiously alike, seeing that the eye-muscles do not develop until 



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THE PROS OM A TIC SEGMENTS OF AMMOCCETES 305 

transformation, but throughout the Ammoccetes stage remain in 
almost as rudimentary a condition as the tentacular muscles. 

Another difficulty with respect to the tentacles is the determina- 
tion of the number of them, owing to the fact that in addition 
to what may be called well-delined tentacles a large number of 
smaller taotile projections are found on the surface of the upper lip, 
as is seen in Fig. 115. In the very young condition, 7 or 8 mm. in 
length, it is easier to make sure on this point. At this stage they 
may be spoken of as arranged in two groups: an anterior small 
group and a posterior larger group. The anterior group consists of 
a pair of very small tentacles and a very small median tentacle, all 
three situated quite dorsally in the front part of the upper lip. The 
posterior group, which is separate from the anterior, consists of five 
pairs of much larger tentacles, the most ventral pair in the mid-line 
ventrally on the lower lip being fused together to form the large 
ventral median tentacle or tongue already mentioned. This pair, 
according to Shipley, is markedly larger than the others. There are, 
therefore, five conspicuous tentacles on each side, and in front of 
them a smaller pair and a small median dorsal one. In the very 
young condition the accessory projections above-mentioned are not 
present, or at all events are not conspicuous, and the tentacles are 
also markedly larger in comparison to the size of the animal than 
in the older condition, where they have distinctly dwindled. 

This posterior group of five conspicuous tentacles is the one which 
I suggest represents the four endognaths and one ectognath. What 
the significance of the small anterior group is, I know not. It is pos- 
sible that the chelicerse are represented here, for they are situated 
distinctly anterior to the other group ; I know, however, of no sign of 
a markedly separate innervation to these most dorsal tentacles such 
as I should have expected to find if they represented the chelicene. 

The muscles of the upper lip, which distinctly belong to the 
visceral and not to the somatic musculature, form part of the fore- 
most segments, and in these muscles the teutacular nerve reaches its 
final destination. From their innervation, then, they must have 
belonged to the same appendages as the tentacles supplied by the 
tentacular nerve, i.e. to the endognaths. What conclusion can we 
form as to the probable origin of the upper lip of Ammoccutes ? 
Since the oral chamber was formed by the forward growth of the 
metastoma, i.e. the lower lip of Ammocoetes, it follows that the upper 

x 



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306 THE ORIGIN OF VERTEBRATES 

lip is the continuation forwards of the original ventral surface of such 
an animal as Limulus or a member of the scorpion group, where there 
is no metastoma, and corresponds to the endostoma, as Holm calls it, 
of Eurypterus. This termination of the ventral surface in all these 
animals is made up of two parts : (1) Of sternites composing the true 
median ventral surface of the body, called by Lankester the pro- 
and meso-sternites ; and (2) of the sterno-coxal processes of the fore- 
most prosomatic appendages, called in the case of Limulus gnathites, 
because they are the main agents in triturating the food previously 
to its passage into the mouth. In Limulus, a conjoined pro-meso- 
sternite forms the median ventral wall to which the sterno-coxal 
processes are attached on each side, and in Phrynus and Mygale a 
well-marked pro-sternite and meso-sternite are present, forming the 
posterior limit of the olfactory opening. In Buthus and the true 
scorpions the sterno-coxal processes of the 2nd, 3rd, and 4th pro- 
somatic appendages take part in surrounding the olfactory tubular 
passage ; in Thelyphonus only the processes of the 2nd pair of pro- 
somatic appendages play such a part, the pro-sternite not being 
present (cf. Fig. 97). 

Seeing, then, what a large share the sterno-coxal processes of one 
or more of these prosomatic appendages plays in the formation of 
this endostoma, and seeing also that the nerve which supplies the 
upper lip-muscles in Ammoccetes is the same as that supplying the 
tentacles which are attached to the upper lip, it appears to me more 
probable than not that the muscles in question are the vestiges of 
the sterno-coxal muscles. These muscles differ markedly in their 
attachments from the muscles of the lower lip, for whereas the latter 
resemble the tergo-coxal group in their extreme dorsal attachment, 
the former resemble the sterno-coxal group in their attachment to 
what corresponds to the endostoma. 

This interpretation of the meaning of the transformation process 
is in accordance with all the previous evidence both from the side 
of the palaeostracan as from the side of the vertebrate, for it signifies 
that a dwindling process has taken place in the foremost of the 
original prosomatic appendages — the chelicerse and the endognaths ; 
while, on the contrary, the ectognath and the metastoma have con- 
tinued to increase in importance right into the vertebrate stage. 
This process is simply a continuation of what was already going on 
in the invertebrate stage, for whereas in Eurypterus and other cases 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 307 

the chelicerae and endognaths had dwindled down to mere tentacles, 
the ectognath was the large swimming appendage, and the metas- 
toma was on the upward grade from the two insignificant chilaria of 
Limulus. 

The transformation of these foremost appendages into a suctorial 
apparatus is very common among the arthropods, as is seen in the 
transformation of the caterpillar into the butterfly, and it is in 
accordance with the evidence that the main mass of that suctorial 
apparatus should be formed from appendages corresponding to the 
ectognath and metastoma rather than from the four endognaths. In 
all probability the nucleus masticatorius of the trigeminal nerve with 
its innervation of the great muscles of mastication is evidence of the 
continued development of the musculature of these two last pro- 
somatic appendages, just as the descending root of the Vth demon- 
strates the further disappearance of all that belongs to the foremost 
prosomatic appendages. As yet, however, as far as I know, the 
musculature of the head-region of Petromyzon has not been brought 
into line with that of other vertebrates, and until that comparative 
study has been completed it is premature to discuss the exact posi- 
tion of the masticating muscles of the higher vertebrates. 

The analysis of these tentacular segments belonging to the 
trigeminal nerve presents greater difficulties than that of any of the 
other cranial segments, owing to the deficiency of our knowledge 
of what occurs at transformation. Light is required not only on the 
origin of the new muscles but also on the origin of the new and 
elaborate cartilages which are newly formed at this time. 

Miss Alcock has not yet worked out the origin of all these carti- 
lages and muscles, so that we are not yet in a position to analyze 
the trigeminal supply in Petromyzon into its component appendage 
elements, an analysis which ought ultimately to enable us to deter- 
mine from which appendage-muscles the masticating muscles in the 
higher vertebrates have arisen. As far as the muscles are con- 
cerned, she gives me the following information : — 

The tongue-nerve supplies in Ammocu'tes the rudimentary 
muscles which pass laterally from the base of the large ventral 
tentacle to the wall of the throat, and even in Ammoccetes must 
possess some power of moving that tentacle. 

At transformation these muscles proliferate and develop enor- 
mously, and form the bulk of the large basilar muscle which 



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3 o8 



THE ORIGIN OF VERTEBRATES 



surrounds the throat ventrally and laterally, and is the most bulky 
muscle in the suctorial apparatus. 

The velar or mandibular nerve supplies in Ammoccetes the 
muscles of the lower lip. In Petromyzon it supplies also the 
longitudinal muscles of the tongue. The tongue-cartilage first 
develops in the region of the median ventral tentacle, and there 
the longitudinal tongue-muscles first begin to develop, not from 
the rudimentary muscles in the tongue but from those in the 
lower lip region. 

In Ammoccetes the tentacular nerve supplies the rudimentary 
muscles in the tentacles and the muscles of the upper lip. The 
latter disappear entirely at transformation, and in Petromyzon the 
tentacular nerve supplies the circular, pharyngeal, and annular 
muscles, which are derived from the rudimentary tentacular 
muscles. 

For the convenience of my reader I append here a table showing 
my conception of the manner in which the endognathal and ecto- 
gnathal segments of the Palceostracan are represented in Ammoccetes. 
It shows well the uniform manner in which all the individual 
segmental factors have been fused together to represent the appear- 
ance of a single segment (van Wijhe's first segment) in the case of 
the four endognathal segments, but have retained their individuality 
in the case of the ectognathal segment. 



• 3 ts 
\a.a So 



Appendages. 



Appendage 
nerves. 



Eurypterid. Ammoccetes 



4 Endo- 
gnaths 



1 Ecto- 
gnath 



Skeletal 
elements. 



Somatic 
motor 



4 Ten- 
tacles 



1 Ten- 1 1 Ten- 
tacular tacular 
to 4 bar to 4 
tentacles tentacles 



Dorso- 

ventral Ccelomlc 
segmental cavities, 
muscles. 



Cozal 
glands. 



lOculo- Sup. ' lPre- I 1 ?' 1 "" 

motor inf. int. \ mandi- /*££ . ' 

supply- rectus bular L . ^'. 

ing4 and inf. fusion ^icnof 

muscles oblique of 4 j *™»~ 



1 Troch- 

1 Tongue 1 Tongue j 1 Tongue |£™ Sup. IMan- j 
6 nerve | bar , j ** £ oblique dibular | 

i , muscle 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 309 



The Tubular Muscles. 

The only musculature innervated by the trigeminal nerve which 
remains for further discussion, consists of those peculiar muscles found 
in the velum, known by the name of striated tubular muscles. This 
group of muscles has already been referred to in Chapter IV., dealing 
with respiration and the origin of the heart. 

It is a muscular group of extraordinary interest in seeking an 
answer to the question of vertebrate ancestry, for, like the thyroid 
gland, it bears all the characteristics of a 
survival from a prevertebrate form, which 
is especially well marked in Ammoccetes. 
I have already suggested in this chapter 
that the homologues of these muscles are 
represented in Limulus by the veno-peri- 
cardial group of muscles. I will now 
proceed to deal with the evidence for this 
suggestion. 

The structure of the muscle-fibres is 
peculiar and very characteristic, so that 
wherever they occur they are easily recog- 
nized. Each fibre consists of a core of 
granular protoplasm, in the centre of which 
the nuclei are arranged in a single row. 
This core is surrounded by a margin of 
striated fibrillar, as is seen in Fig. 122. 
Such a structure is characteristic of various 
forms of striated muscle found in various 

invertebrates, such as the muscle-fibre of mollusca. It is, as far as 
I know, found nowhere in the vertebrate kingdom, except in Ammo- 
cfotes. At transformation these muscles entirely disappear, becoming 
fattily degenerated and then absorbed. 

For all these reasons they bear the stamp of a survival from a 
prevertebrate form. This alone would not make this tissue of any 
great importance, but when in addition these muscles are found to be 
arranged absolutely segmeutally throughout the whole of the branchial 
region, then tliis tissue becomes a clue of the highest importance. 

As mentioned in Chapter IV., the segmental muscles of respira- 
tion consist of the adductor muscle and the two constrictor muscles 




Fia. 122. — A Tubular 
Muscle-fibre of Ammo- 
ccetes. 

A, portion of fibre seen longi- 
tudinally ; B, transverse 
section of fibre (osmic pre- 
paration) ; the black dots 
are fat-globules. 



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3IO THE ORIGIN OF VERTEBRATES 

— the striated constrictor and the tubular constrictor. Of these 
muscles, both the muscles possessing ordinary striation are attached 
to the branchial cartilaginous skeleton, whereas the tubular con- 
strictors have nothing to do with the cartilaginous basket-work, but 
are attached ventrally in the neighbourhood of the ventral aorta. 

These segmental tubular muscles are found also in the velar folds 
— the remains of the septum or velum which originally separated 
the oral from the respiratory chamber. In the branchial region they 
act with the other constrictors as expiratory muscles, forcing the 
water out of the respiratory chamber. In the living Ammoccetes, 
the velar folds on each side can be seen to move synchronously with 
the movements of respiration, contracting at each expiration; they 
thus close the slit by which the oral and respiratory chambers com- 
municate, and therefore, in conjunction with the respiratory muscles, 
force the water of respiration to flow out through the gill-slits, as 
described by Schneider. 

These tubular muscles thus form a dorso- ventral system of 
muscles essentially connected with respiration ; they belong to each 
one of the respiratory segments, and are also found in the velum ; 
anterior to this limit they are not to be found. What, then, are these 
tubular muscles in the velar folds ? Miss Alcock has worked out 
their topography by means of serial sections, and, as already fully 
explained, has shown that they form exactly similar dorso- ventral 
groups, which belong to the two segments anterior to the purely 
branchial segments, i.e. to the facial or hyoid segments and the lower 
lip-segment of the trigeminal nerve. If the velar folds could be put 
back into their original position as a septum, then the hyoid or facial 
group of tubular muscles would take up exactly the same position as 
those belonging to each branchial segment. 

The presence of these two so clearly segmental groups of muscles 
in the velum — the one belonging to the region of the trigeminal, the 
other to the region of the facial — is strong confirmation of my con- 
tention that this septum between the oral and respiratory chambers 
was caused by the fusion of the last prosomatic and the first meso- 
somatic appendages, represented in Limulus by the chilaria and the 
operculum. 

Yet another clue to the meaning of these muscles is to be found 
in their innervation, which is very extraordinary and unexpected. 
Throughout the branchial region the striated muscles of each segment 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 3II 

are strictly supplied by the nerve of that segment, and, as already 
described, each segment is as carefully mapped out in its innervation 
as it is in any arthropod appendage. One exception occurs to this 
orderly, symmetrical arrangement : a nerve arises in connection with 
the facial nerve, and passes tailwards throughout the whole of the 
branchial region, giving off a branch to each segment as it passes. 
This nerve {Br. pro/., Fig. 123) is known by the name of the ramus 
branehialis profundus of the facial, and its extraordinary course has 
always aroused great curiosity in the minds of vertebrate anatomists. 
Miss Alcock, by the laborious method of following its course through- 
out a complete series of sections, finds that each of the segmental 
branches which is given off, passes into the tubular muscles of that 
segment (Fig. 124). The tubular muscles which belong to the velum, 



r.IUe VII 



n L<tt VII -X 




lt.Hy. ».TKj. 

Fig. 123.— Diagram showing the Distribution of the Facial Nerve. 
Motor branches, red ; sensory branches, blue. 

i.e. those belonging to the lower lip-segment and to the hyoid segments, 
receive their innervation from the velar or mandibular nerve, and 
belong, therefore, to the trigeminal, not to the facial, system. 
The evidence presented by these muscles is as follows : — 
In the ancestor of the vertebrate there must have existed a seg- 
mentally arranged set of dorso-ventral muscles of peculiar structure, 
concerned with respiration, and confined to the mesosomatic segments 
and to the last prosomatic segment, yet differing from the other 
dorso-ventral muscles of respiration in their innervation and their 
attachment. 

Interpreting these facts with the aid of my theory of the origin 
of vertebrates, and remembering that the homologue of the vertebrate 
ventral aorta in such a paheostracan as Limulus is the longitudinal 



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312 



THE ORIGIN OF VERTEBRATES 



venous sinus, while the opercular and chilarial segments are respec- 
tively the foremost mesosomatic and the last prosomatic segments; 
they signify that the palaeostracan ancestor must have possessed a 
separate set of segmental dorsoventral muscles confined to the bran- 
chial, opercular and chilarial or metastomal segments, which, on the 








Fig. 124.— Diagram constructed from a series of Transverse Sections through 
a Branchial Segment, showing the arrangement and relative positions 
of the Cartilage, Muscles, Nerves, and Blood- Vessels. 

Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured 
blue are the internal sensory nerves to the diaphragms and the external sensory 
nerves to the sense-organs of the lateral line system. Br. cart., branchial 
cartilage; M. con. str. } striated constrictor muscles; M. con. tub., tubular 
constrictor muscles ; M. add., adductor muscle ; D.A., dorsal aorta ; V.A., ventral 
aorta; .S., sense-organs on diaphragm; n. Lat., lateral line nerve; A"., epibran- 
chial ganglia of vagus; R. br. prof. VII. , ramus branchialisprofundus of facial ; 
J.v., jugular vein ; Ep. pit., epithelial pit. 

one hand, were respiratory in function, and on the other were attached 
to the longitudinal venous sinus. Further, these muscles must all 
have received a nerve-supply from the neuromeres belonging to the 
chilarial and opercular segments, an unsymmetrical arrangement of 
nerves, on the face of it, very unlikely to occur in an arthropod. 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 313 

Is this prophecy borne out by the examination of Limulus ? In 
the first place, these muscles were dorso ventral and segmental, and, 
referring back to Chapter VII., Lankester arranges the segmental 
dorso-ventral muscles in three groups : (1) The dorso-ventral somatic 
muscles ; (2) the dorso-ventral appendage muscles ; and (3) the veno- 
pericardial muscles. Of these the first group is represented in the 
vertebrate by the muscles which move the eye, the second group by 
the striated constrictor and adductor muscles and the muscles for the 
lower lip. There is, then, the possibility of the third group for this 
system of tubular muscles. 

Looking first at the structure of these muscles as previously de- 
scribed, so different are they in appearance from the ordinary muscles 
of Limulus, that Milne-Edwards, as already stated, called them 
" brides transparentes," and did not recognize their muscular cha- 
racter, while Blanchard called them in the scorpion, "ligaments 
contractus." 

Consider their attachment and their function. They are attached 
to the longitudinal sinus, according to Lankester's observation, in such 
a way that the muscle-fibres form a hollow cone filled with blood ; 
when they contract they force this blood towards the gills, and thus 
act as accessory or branchial hearts. According to Blanchard, in the 
scorpion they contract synchronously with the heart ; according to 
Carlson, in Limulus they contract with the respiratory muscles. In 
Ammoccetes, where the respiration is effected after the fashion of 
Limulus, not of Scorpio, the tubular muscles are respiratory in 
function. 

Look at their limits. The veno-pericardial muscles in Limulus 
are limited by the extent of the heart, they do not extend beyond 
the anterior limit of the heart. In Fig. 70 (p. 176) two of these 
muscles are seen in front of the branchial region also attached to the 
longitudinal venous sinus, although in front of the gill-region. In 
Ammoccetes the upper limit of the tubular muscles is the group 
found in the velum ; this most anterior group belongs to a region in 
front of the branchial region — that of the trigeminal. 

Moreover, the supposition that the segmental tubular muscles 
belong throughout to the veno-pericardial group gives an adequate 
reason why they do not occur in front of the velum ; for, as their 
existence is dependent upon the longitudinal collecting sinus in 
Limulus and Scorpio, which is represented by the ventral aorta in 



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3 H THE ORIGIN OF VERTEBRATES 

Ammoccetes, they cannot extend beyond its limits. Now, Dohrn 
asserts that the ventral aorta terminates in the spiracular artery, 
which exists only for a short time ; and, in another place, speaking 
of this same termination of the ventral aorta, he states : " Dass je 
eine vorderste Arterie aus den beiden primaren Aesten des Conus 
arteriosus hervorgeht, die erste Anlage der Thyroidea umfasst, in der 
Mesodermfalte des spateren Velums in die Hohe steigt urn in die 
Aorta der betreffenden Seite einzumunden." These observations 
show that the vessel which in Ammoccetes represents the longitudinal 
collecting sinus in the Merostomata does not extend further forwards 
than the velum, and in consequence the representatives of the veno- 
pericardial muscles cannot extend into the segments anterior to the 
velum. One of the extraordinary characteristics of these tubular 
muscles which distinguishes them from other muscles, but brings them 
into close relationship with the veno-pericardial group, is the manner 
in which the bundles of muscle-fibres are always found lying freely 
in a blood-space ; this is clearly seen in the branchial region, but 
most strikingly in the velum, the interior of which, apart from its 
mueo-cartilage, is simply a large lacunar blood-space traversed by 
these tubular muscles. 

All these reasons point to the same conclusion : the tubular 
muscles in Ammoccetes are the successors of the veno-pericardial 
system of muscles. 

If this is so, then this homology ought to throw light on the 
extraordinary innervation of these tubular muscles by the branchialis 
profundus branch of the facial nerve and the velar branch of the 
trigeminal. We ought, in fact, to find in Limulus a nerve arising 
exclusively from the ganglia belonging to the chilarial and opercular 
segments, which, instead of being confined to those segments, traverses 
the whole branchial region on each side, and gives off a branch to 
each branchial segment; this branch should supply the veno-peri- 
cardial muscle of that side. 

Patten and Eedenbaugh have traced out the distribution of the 
peripheral nerves in Limulus, and have found that from each meso- 
somatic ganglion a segmental cardiac nerve arises which passes to 
the heart and there joins the cardiac median nerve, or rather the 
median heart-ganglion, for this so-called nerve is really a mass of 
ganglion-cells. In all the branchial segments the same plan exists, 
each cardiac nerve belonging to that neuromere is strictly segmental. 



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THE PROSOMATIC SEGAfENTS OF AMMOCCETES 315 

Upon reaching the opercular and chilarial neuromeres an extra- 
ordinary exception is found ; the cardiac nerves of these two neuro- 
meres are fused together, run dorsally, and then form a single nerve 
called the pericardial nerve, which runs outside the pericardium 
along the whole length of the mesosomatic region, and gives off a 
branch to each of the cardiac nerves of the branchial neuromeres as 
it passes them. 

This observation of Patten and Eedenbaugh shows that the peri- 
cardial nerve of Limulus agrees with the very nerve postulated by 
the theory, as far as concerns its origin from the chilarial and 
opercular neuromeres, its remarkable course along the whole 
branchial region, and its segmental branches to each branchial 
segment. 

At present the comparison goes no further ; there is no evidence 
available to show what is the destination of these segmental branches 
of the pericardial nerve, and so far all evidence of their having any 
connection with the veno-pericardial muscles is wanting. Carlson, 
at my request, endeavoured in the living Limulus to see whether 
stimulation of the pericardial nerve caused contraction of the veno- 
pericardial muscles, but was unable to find any such effect. On the 
contrary, his experimental work indicated that each veno-pericardial 
muscle received its motor supply from the corresponding mesosomatic 
ganglion. This is not absolutely conclusive, for if, as Blanchard 
asserts in the case of the scorpion, a close connection exists between 
the action of these muscles and of the heart, it is highly probable 
that their innervation conforms to that of the heart. Now Carlson 
has shown that this cardiac nerve from the opercular and chilarial 
neuromeres is an inhibitory nerve to the heart, while the segmental 
cardiac nerves belonging to the branchial ganglia are the augmentor 
nerves of the heart. 

His experiments, then, show that the motor nerves of the heart 
and of the veno-pericardial muscles run together in the same nerves, 
but he says nothing of the inhibitory nerves to the latter muscles. 
If they exist and if they are in accordance with those to the heart, 
then they ought to run in the pericardial nerve, and would naturally 
reach the veno-pericardial muscles by the segmental branches of the 
pericardial nerve. 

Moreover, inhibitory nerves are, in certain cases, curiously 
associated witli sensory fibres ; so that the nerve which corresponds 



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316 THE ORIGIN OF VERTEBRATES 

to the pericardial nerve, viz. the branchialis profundus of the facial, 
may be an inhibitory and sensory nerve, and not motor at all. Miss 
Alcock's observations are purely histological ; no physiological 
experiments have been made. 

At present, then, it does not seem to me possible to say that 
Carlson's experiments have disproved any connection of the peri- 
cardial nerve with the veno-pericardial muscles. We do not know 
what is the destination of its segmental branches ; they may still 
supply the veno-pericardial muscles even if they do not cause them 
to contract; they certainly do not appear to pass directly into them, 
for they pass into the segmental cardiac nerves, and can only reach 
the muscles in conjunction with their motor nerves. Such a course 
would not be improbable when it is borne in mind how, in the frog, 
the augmentor nerves run with the inhibitory along the whole length 
of the vagus nerve. 

Until further evidence is given both as to the function of the seg- 
mental branches of the pericardial nerve in the Limulus, and of the 
branchialis profundus in Ammocoetes, it is impossible, I think, to 
consider that the phylogenetic origin of these tubular muscles is as 
firmly established as is that of most of the other organs already 
considered. I must say, my own bias is strongly in favour of looking 
upon them as the last trace of the veno-pericardial system of muscles, 
a view which is distinctly strengthened by Carlson's statement that 
the latter system contracts synchronously with the respiratory move- 
ments, for undoubtedly in Ammocoetes their function is entirely 
respiratory. Then again, although at present there is no evidence to 
connect the pericardial nerve in Limulus with this veno-pericardial 
system of muscles, yet it is extraordinarily significant that in such 
animals as Limulus and Ammocoetes, in both of which the mesoso- 
matic or respiratory region is so markedly segmental, an intrusive 
nerve should, in each case, extend through the whole region, giving 
off branches to each segment. Still more striking is it that this 
nerve should arise from the foremost mesosomatic and the last pro- 
somatic neuromeres in Limulus — the opercular and chilarial segments 
— precisely the same neuromeres which give origin to the correspond- 
ing nerve in Ammocoetes, for according to my theory of the origin of 
vertebrates, the nerves which supplied the opercular and metastomal 
appendages have become the facial nerve and the lower lip-branch 
of the trigeminal nerve. 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 317 

With the formation of the vertebrate heart from the two longi- 
tudinal venous sinuses and the abolition of the dorsal invertebrate 
heart, the function of these tubular muscles as branchial hearts was 
no longer needed, and their respiratory function alone remained. The 
last remnant of this is seen in Ammoccetes, for the ordinary striated 
muscles were always more efficient for the respiratory act, and so at 
transformation the inferior tubular musculature was got rid of, there 
being no longer any need for its continued existence. 

The Pal^iostoma, or Old Mouth. 

The arrangement of the oral chamber in Ammoccetes is peculiar 
among vertebrates, and, upon my theory, is explicable by its 
comparison with the accessory oral chamber which apparently 
existed in Eurypterus. According to this explanation, the lower lip 
of the original vertebrate mouth was formed by the coalescence of 
the most posterior pair of the prosomatic appendages — the chilaria ; 
from which it follows that the vertebrate mouth was not the original 
mouth, but a new structure due to such a formation of the lower lip. 

It is very suggestive that the direct following out of the original 
working hypothesis should lead to this conclusion, for it is universally 
agreed by all morphologists that the present mouth is a new forma- 
tion, and Dohrn has argued strongly in favour of the mouth being 
formed by the coalescence of a pair of gill-slits. Interpret this in 
the language of my theory, and immediately we see, as already 
explained, gill-slits must mean in this region the spaces between 
appendages which did not carry gills; the mouth, therefore, was 
formed by the coalescence of a pair of appendages to form a lower 
lip just as I have pointed out. 

Where, then, must we look for the palaeostoma, or original mouth ? 
Clearly, as already suggested, it was situated at the base of the olfac- 
tory passage, and the olfactory passage or nasal tube of Ammoccetes 
was originally the tube of the hypophysis, so that the following out 
of the theory points directly to the tube of the hypophysis as the 
place where the palaeostoma must be looked for. 

This conclusion is not only not at variance with the opinions of 
morphologists, but gives a straightforward, simple explanation why 
the palseostoma was situated in the very place where they are most 
inclined to locate it. Thus, if we trace the history of the question, 



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[8 



THE ORIGIN OF VERTEBRATES 



we see that Dohrn's original view of the comparison of the vertebrate 
and the annelid led him to the conception that the vertebrate mouth 
was formed by the coalescence of a pair of gill-slits, and that the 
original mouth was situated somewhere on the dorsal surface and 
opened into the gut by way of the infundibulum and the tube of the 
hypophysis. This, also, was Cunningham's view as far as the tube 
of the hypophysis was concerned. Beard, in 1888, holding the view 
that the vertebrates were derived from annelids which had lost their 
supra- oesophageal ganglia, and that, therefore, there was no question 
of an oesophageal tube piercing the central nervous system of the 
vertebrate, explained the close connection of the infundibulum with 
the hypophysis by the comparison of the tube of the hypophysis with 



l|-.f..VJ,n'itv^,C.-.....- | ^ 




^:^^^^^!^vwJ ! l■■',.; ^ K ■ w^. ^ .'» ■ ^ ^ ^^IJJ i . l■l uju mji M M 

■ ■:.-■. :'-;V:: : .: 



c. 
Nch. 



Fig. 125.— Diagram to show the Meeting of the Four Tubes in such a 
Vertebrate as the Lamprey. 

iyc, neural canal with its infundibular termination ; Nch., notochord ; AL, alimentary 
canal with its anterior diverticulum ; Hy., hypophysial or nasal tube ; Or., oral 
chamber closed by septum. 



the annelidan mouth, so that the infundibular or so-called nervous 
portion was a special nervous innervation for the original throat, 
just as Kleinenberg had shown to be the case in many annelids. 
Beard therefore called this opening of the hypophysial tube the old 
mouth, or pakeostoma. Recently, in 1893, Kupffer has also put 
forward the view that the hypophysial opening is the palaeostoma, 
basing this view largely upon his observations on Ammocoetes and 
Acipenser. 

As is seen in Fig. 125, the position of this palseostoma is a very 
suggestive one. At this single point in Ammocoetes, four separate 
tubes terminate ; here is the end of the notochordal tube, the termina- 
tion of the infundibulum, the blind end of the nasal tube or tube 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 319 

of the hypophysis, and the pre-oral elongation of the alimentary 
canal. 

It is perfectly simple and easy for the olfactory tube to open into 
any one of the other three. By opening into the infundibulum it 
reproduces the condition of affairs seen in the scorpion ; by opening 
into the gut it produces the actual condition of things seen in 
Myxine and other vertebrates ; by opening into the notochordal tube 
it would produce a transitional condition between the other two. 

The view held by Kupffer is that this nasal tube- (tube of the 
hypophysis) opened into the anterior diverticulum of the vertebrate 
gut, and was for this reason the original mouth-tube ; then a new 
mouth was formed, and this connection was closed, being subse- 
quently reopened as in Myxine. My view is that this tube 
originally opened into the infundibulum, in other words, into the 
original gut of the palaeostracan ancestor, and was for this reason 
the original mouth-tube, in the same sense as the olfactory passage 
of the scorpion may be, and often is, called the mouth-tube. When, 
with the breaking through of the septum between the oral and 
respiratory chambers, the external opening of the oral chamber 
became a new mouth, the old mouth was closed but the olfactory 
tube still remained, owing to the importance of the sense of smell. 
Subsequently, as in Myxine and the higher vertebrates, it opened 
into the pharynx, and so formed the nose of the higher vertebrates. 

It is not, to my mind, at all improbable that during the transition 
stage, between its connection with the old alimentary canal, as in 
Eurypterus or the scorpions, and its blind ending, as in Ammoccetes, 
the nasal tube opened into the tube of the notochord. This question 
will be discussed later on when the probable significance of the 
notochord is considered. 

The Pituitary Gland. 

Turning back to the comparison of Fig. 106, B, and Fig. 106, C, 
which represent respectively an imaginary sagittal section through 
an Eurypterus-like animal and through Ammoccetes at a larval 
stage, all the points for comparison mentioned on p. 244 have now 
been discussed with the exception of the suggested homology 
betweeu the coxal glands of the one animal and the pituitary 
body of the other. 



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320 THE ORIGIN OF VERTEBRATES 

This latter gland undoubtedly arises posteriorly to the hypophysial 
tube, or Rathke's pouch (as it is sometimes called), and, as already 
mentioned, is supposed by Kupffer to be formed from the posterior 
wall of this pouch. More recently, as pointed out in Haller's paper, 
Nusbaum, who has investigated this matter, finds that the glandular 
hypophysis is not formed from the walls of Rathke's pouch, but from 
the tissue of the rudimentary connection or stalk between the two 
premandibular cavities, which becomes closely connected with the 
posterior wall of Rathke's pouch, and becoining cut off from the 
rest of the premandibular cavity on each side, becomes permanently 
a part of the ' Hypophysis Anlage.' 

The importance of Nusbaum's investigation consists in this, that 
he derives the glandular hypophysis from the connecting stalk 
between the two premandibular cavities, and therefore from the 
walls of the ventral continuation of this cavity on each side. 

This may be expressed as follows : — 

The coelomic cavity, known as the premandibular cavity, divides 
into a dorsal and a ventral part ; the walls of the dorsal part give 
origin to the somatic muscles belonging to the oculomotor nerve, 
while the walls of the ventral part on each side form the connecting 
stalk between the two cavities, and give origin to the glandular 
hypophysis. 

Now, as already pointed out, the premandibular cavity is homo- 
logous with the 2nd prosomatic coelomic cavity of Limulus, and this 
2nd prosomatic coelomic cavity divides, according to Kishinouye, into 
a dorsal and a ventral part ; and, further, the walls of this ventral 
part form the coxal gland. Both in the vertebrate, then, and in 
Limulus, we find a marked glandular tissue in a corresponding 
position, and the conclusion is forced upon us that the glandular 
hypophysis was originally the coxal gland of the invertebrate an- 
cestor. As in all other cases already considered, when the faots of 
topographical anatomy, of morphology and of embryology, all com- 
bine to the same conclusion as to the derivation of the vertebrate 
organ from that of the invertebrate, then there must be also a struc- 
tural similarity between the two. What, then, is the nature of the 
coxal gland in the scorpions and Limulus ? Lankester's paper gives 
us full information on this point as far as the scorpion and Limulus 
are concerned, and he shows that the coxal gland of Limulus differs 
markedly from that of Scorpio in the size of the cells and in the 



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THE PROSOMATIC SEGMENTS OF AMMOCiETES 32 1 

arrangement of the tubes. In Fig. 126, A, I give a picture of a piece 
of the coxal gland of Limulus taken from Lankester's paper. 

Turning now to the vertebrate, Bela Haller's paper gives us a 
number of pictures of the glandular hypophysis from various verte- 
brates, and he especially points out the tubular nature of the gland 
and its solidification in the course of development in some cases. 
In Fig. 126, B, I give his picture of the gland in Ammocoetes. 

The striking likeness between Haller's picture and Lankester's 
picture is apparent on the face of it, and shows clearly that the 
histological structure of the glands in the two cases confirms the 
deductions drawn from their anatomical and morphological positions. 



3KB£§8KJ£*a*» 







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':? 



Fig. 126. — A, Section op Coxal Gland of Limulus (from Lankester) ; B, 
Section op Pituitary Body op Ammocxetes (from Bela Haller\ 

n.a., termiuatiou of nasal passage. 



The sequence of events which gave rise to the pituitary body 
of the vertebrate was in all probability somewhat as follows : — 

Starting with the excretory glands of the Phyllopoda, known as 
shell-glands, which existed almost certainly in the phyllopod Trilo- 
bite, we pass to the coxal gland of the Merostomata. Judging from 
Limulus, these were coextensive with the coxse of the 2nd, 3rd, 4th, 
and 5th locomotor appendages. When these appendages became 
reduced in size and purely tactile they were compressed and con- 
centrated round the mouth region, forming the endognaths of the 
Merostomata ; as a necessary consequence of the concentration of the 
coxae of the endognaths, the coxal gland also became concentrated, 

Y 




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322 THE ORIGIN OF VERTEBRATES 

and took up a situation close against the pharynx, as represented in 
Fig. 106, B. When, then, the old mouth closed, and the pharynx 
became the saccus vaseulosus, the ooxal gland remained in close 
contact with the saccus vasculosus, and became the pituitary body, 
thus giving the reason why there is always so close a connection 
between the pituitary body and the infundibular region. 

Whatever was the condition of the digestive tracts at the transi- 
tion stage between the arthropod and the vertebrate, the original 
mouth-opening at the base of the olfactory tube was ultimately 
closed. The method of its closure was exceedingly simple and 
evident. The membranous cranium was in process of formation by 
the extension of the plastron laterally and dorsally ; a slight growth 
of the same tissue in the region of the mouth would suffice to close 
it and thus separate the infundibulum from the olfactory tube. As 
evidence that such was the method of closure, it is instructive to 
see how in Ammoccetes the glandular tissue of the pituitary body 
is embedded in and mixed up with the tissue of this cranial wall ; how 
the termination of the nasal tube is embedded in this same thickened 
mass of the cranial wall — how, in fact, both coxal gland and olfac- 
tory tube have become involved in the growth of the tissue of the 
plastron, by means of which the mouth was closed. 

I have now passed in review the nature of the evidence which 
justifies a comparison between the segments supplied by the cranial 
nerves of the vertebrate and the prosomatic and mesosomatic segments 
of the paleeostracan. For the convenience of my readers I have put 
these conclusions into tabular form (see p. 323), for all the segments as 
far as that supplied by the glossopharyngeal nerves. In both verte- 
brate and invertebrate this is a fixed position, for in the former, how- 
ever variable may be the number of branchial segments which the 
vagus supplies, the second branchial segment is always supplied by a 
separate nerve, the glossopharyngeal, and in the latter, though the 
number of segments bearing branchiae varies, the minimum number 
of such segments (as seen in the Pedipalpi) is never less than two. 



Summary. 

The general consideration of the evidence of the number of segments, and 
their nature in the pro-otic region of the vertebrate, as given in the last 
chapter, is not incompatible with the view that the trigeminal nerve originally 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 323 



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Digitized by VjOOQIC 




324 THE ORIGIN OF VERTEBRATES 

supplied seven appendages, which appendages did not carry branchiae, but were 
originally used for purposes of locomotion as well as of mastication. 

Such appendages clearly no longer exist in the higher vertebrates, the 
muscles of mastication only remaining ; but in the earliest fish-forms they must 
have existed, as. indeed, is seen in Ptericthys and Bothriolepis. Judging from 
all the previous evidence some signs of their existence may reasonably be 
expected still to remain in Ammocoetes. Such is indeed the case. 

In the adult Petromyzon the trigeminal nerve innervates specially a 
massive suctorial apparatus, by means of which it holds on to other fishes, or 
to stones in the bottom of the stream. There is here no apparent sign of 
appendages. Very great, however, is the difference in the oral chamber of 
Ammocoetes ; here there is no sign of any suctorial apparatus, but instead, a 
system of tentacles, together with the remains of the septum or velum, which 
originally closed off the oral from the respiratory chamber. These tentacles 
are the last remnants of the original foremost prosomatic appendages of the 
palaeostracan ancestor. Like the lateral eyes they do not develop until the 
transformation comes, but during the whole larval condition their musculature 
remains in an embryonic condition, and then from these embryonic muscles 
the whole massive musculature of the suctorial apparatus develops ; a sucking 
apparatus derived from the modification of appendages, as so frequently occurs 
in the arthropods. 

The study of Ammocoetes indicates that the velum and lower lip correspond 
to the metastoma of the Eurypterid, i.e. the chilaria of Limulus, while the large 
ventral pair of tentacles, called the tongue, correspond to the ectognaths of the 
Eurypterids, and probably to the oar-like appendages of Ptericthys and 
Bothriolepis. From these two splanchnic segments the suctorial apparatus 
in the main arises ; the motor supply of these two segments forms the mass' of 
the trigeminal nerve-supply, and the nerves supplying them, the velar nerve and 
the tongue-nerve, are markedly separate from the rest of the trigeminal nerve. 

The rest of the tentacles present much less the sign of independent 
segments. In their nerves, their muco- cartilaginous skeleton, and their 
rudimentary muscles, they indicate a concentration and amalgamation, such 
as might be expected from the concentrated endognaths. The continuation of 
the dwindling process, already initiated in the Eurypterid, would easily result in 
the tentacles of Ammocoetes. 

The nasal tube of Ammocoetes, which originates in the hypophysial tube, 
corresponds absolutely in position and in its original structure, to the olfactory 
tube of a scorpion-like animal. From this homology two conclusions of 
importance follow: (1) the old mouth, or paheostoma, of the vertebrate was 
situated at the end of this tube, therefore, at the termination of the infundi- 
bulum ; (2) the upper lip, which by its growth, brings the olfactory tube from 
a ventral to a dorsal position, was originally formed by the foremost sternites 
or endostoma, or else by ,the sterno-coxal processes of the second pair of 
prosomatic appendages of the palaeostracan ancestor. 

In strict accordance with the rest of the comparisons made in this region, 
the pituitary body shows by similarity of structure, as well as of position, that 
it arose from the coxal glands, which were situated at the base of the four 
idognaths. 



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THE PROSOMATIC SEGMENTS OF AMMOCCETES 325 

One after another, when once the clue has been found, all these mysterious 
organs of the vertebrate, such as the pituitary and thyroid glands, fall 
harmoniously into their place as the remnants of corresponding important 
organs in the palceostraca. 

Yet another clue is afforded by the tubular muscles of Ammocoetes, that 
strange set of non-vertebrate striated muscles, which are so markedly arranged 
in a segmental manner, which disappear at transformation, and are never found 
in any of the higher vertebrates, for the limits of their distribution correspond 
to the veno-pericardial muscles of Limulus. 

Their nerve-supply in Ammocoetes is most extraordinary; for, although 
they are segmentally arranged throughout the whole respiratory region, which 
is segmentally supplied by the Tilth, IXth, and Xth nerves, and are found in 
front of this region only in one segment, that of the lower lip, which is supplied 
by the velar branch of the Vth nerve, yet they are not supplied segmentally, 
but only by the velar nerve and a branch of the Vllth, the ramus branchiali* 
profundus. This latter nerve extends throughout the respiratory region, and 
gives off segmental branches to supply these muscles. 

It is also a curious coincidence that in such a markedly segmented animal 
as Limulus, a nerve— the pericardial nerve— which arises from the nerves of the 
chilarial and opercular segments, should pass along the whole respiratory region 
and give off branches to each mesosomatic segment. It is strange, to say the 
least of it, that the chilarial or metastomal and the opercular segments of 
Limulus should, on the theory advocated in this book, correspond to the lower 
lip and hyoid segments of the vertebrate. At present the homology suggested 
is not complete, for there is no evidence as yet that the veno-pericardial muscles 
have anything to do with the pericardial nerve. 



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CHAPTER X 

THE RELATIONSHIP OF AMMOCCETES TO THE MOST 
ANCIENT FISHES— THE OSTRACODERMATA 

The nose of the Osteostraci. — Comparison of head-shield of Ammocoates and of 
Cephalaspis. — Ammoccetes the only living representative of these ancient 
fishes. — Formation of cranium. — Closure of old mouth. — Rohon's primordial 
cranium. — Primordial cranium of Phrynus and Galeodes. — Summary. 

The shifting of the orifice of the olfactory passage, which led to the 
old mouth, from the ventral to the dorsal side, as seen in the trans- 
formation of the ventrally situated hypophysial tube of the young 
Ammoccetes, to the dorsally situated nasal tube of the full-grown 
Ammoccetes, affords one of the most important clues in the whole of 
this story of the origin of vertebrates ; for, if Ammoccetes is the 
nearest living representative of the first-formed fishes, then we ought 
to expect to find that the dorsal head-shield of such fishes is differen- 
tiated from that of the contemporary Palteostraca by the presence of 
a median frontal opening anterior to the eyes. Conversely, if such 
median nasal orifice is found to be a marked characteristic of the 
group, in combination with lateral and median eyes, as in Ammoccetes, 
then we have strong reasons for interpreting these head-shields by 
reference to the head of Ammoccetes. 

The oldest known fishes belong to a large group of strange forms 
which inhabited the Silurian and Devonian seas, classed together 
by Smith Woodward under the name of Ostracodermi. These are 
divided into three orders : (1) the Heterostraci, including one family, 
the Pteraspidae, to which Pteraspis and Cyathaspis belong ; (2) the 
Osteostraci, divisible into two families, the Cephalaspidae and Treraa- 
taspidae, which include Cephalaspis, Eukeraspis, Auchenaspis or 
Thyestes, and Tremataspis ; and (3) the Antiarcha, with one family, 
the Astrolepidie, including Astrolepis, Pterichthys, and Bothriolepis. 



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RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 327 

Of these, the first two orders belong to the Upper Silurian, while the 
third is Devonian. 



The Dorsal Head-Shield of the Osteostraci. 

Of the three orders above-named, the Heterostraci and Osteostraci 
are the oldest, and among them the Cephalaspid® have afforded the 
most numerous and best worked-out specimens. At Rootzikull, in 
the island of OKsel, the form known as Thyestes {Auchenaspis) verru- 
cosus is especially plentiful, being found thickly present in among the 
masses of Eurypterid remains, which give the name to the deposit. 
Of late years this species has been especially worked at by Eohon, 
and many beautiful specimens have been figured by him, so that a 
considerable advance has been made in our knowledge since Pander, 
Eichwald, Huxley, Lankester, and Schmidt studied these most 
interesting primitive forms. 

All observers agree that the head-region of these fishes was 
covered by a dorsal and ventral head-shield, while the body-region 
was in most cases unknown, or, as in Eichwald's specimens, and in 
the specimens figured in Lankester and Smith Woodward's memoirs, 
was made up of segments which were not vertebral in character, but 
formed an aponeurotic skeleton, being the hardened aponeuroses 
between the body-muscles. This body- skeleton, which possesses its 
exact counterpart in Ammocoetes, will be considered more fully when 
I discuss the origin of the spinal region of the vertebrate. 

Of the two head-shields, ventral and dorsal, the latter is best 
known and characterizes the group. It consists of a dorsal plate, 
with characteristic horns, which in Thyestes verrucosus (Fig. 128), as 
described by Rohon, is composed of two parts, a frontal part and an 
occipital part (occ.) f the occipital part being composed of segments, 
and possessing a median ridge — the crista occipitalis. In Lankester's 
memoir and in Smith Woodward's catalogue, a large number of known 
forms are described and delineated, and we may perhaps say that in 
some of the forms, such as Eukeraspis pustuliferas (Fig. 127, B), the 
frontal part of the shield only is capable of preservation as a fossil, 
while in Cephalaspis (Fig. 127, A) not only the frontal part but a portion 
of the occipital region is preserved, the latter being small in extent 
when compared with the occipital region of Auchenaspis (Thyestes). 
Finally, in Tremataspis and Didymaspis, the whole of both frontal 



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328 



THE ORIGIN OF VERTEBRATES 



and occipital region is capable of preservation, the line of demarca- 
tion between these two regions being well marked in the latter species. 




p.o.v 




Fig. 127.— A, Dorsal Head-Shield of Cephalaspis (from Lankester) ; B, Dorsal 
Head-Shield of Keraspis (from Lankester). 

In the best preserved specimens of all this group of fishes a frontal 
median orifice is always present; it appears in some specimens 

obscurely partially divided into 
two parts. Perhaps the best 
specimen of all was obtained 
by Eohon at Eootzikiill, and 
is thus described by him : — 

The frontal part of the dorsal 
head-plate carried (Fig. 128) the 
two orbits for the lateral eyes 
(I.e.), a marked frontal organ 
(fro.), and a median depression 
(#/.), to which he gives the 
name parietal organ. The oc- 
cipital part (occ.) was clearly 
segmented, and carried, he 
thinks, the branchia). 1 repro- 
duce Rohon's figure of the 
frontal organ in Thyestes (Fig. 
129) ; he describes it as a 
deeply sunk pit, divided in the middle by a slit, which leads deeper 
in, he supposes, towards the central nervous system. 




Fig. 128. —Dorsal Head-Shield of Thy- 
estes (AiichcnaAjns) verrucosus. (From 
Rohon.) 

Fro., narial opening ; I.e., lateral eyes ; gl, 
glabellum or plate over brain ; Occ, oc- 
cipital region. 



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RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 329 



A similar organ was described by Schmidt in Tremataspis, and 
considered by him to be a median nose. Such also is the view of 
Jaekel, who points out that a median 
pineal eye exists between the two 
lateral eyes in this animal, as in all 
other of these ancient fishes, so that 
this frontal organ does not, as Patten 
thinks, represent the pineal eye. The 
whole of this group of fishes, then, is 
characterized by the following striking 
characteristics : — 

1. Two well-marked lateral eyes 
near the middle line. 

2. Between the lateral eyes, well- 
marked median eyes, very small 

3. In front of the eye-region a median orifice, single. 

In addition, behind the eye-region a median plate is always found, 
frequently different in structure to the rest of the head-shield, being 
harder in texture — the so-called post-orbital plate. 




Fig. 129. — Narial Opening and 
Lateral Orbits of ThyesUs 
Verrucosus. (From Rohon.) 



Structure of Head-Shield of Cephalaspis compared with that 

of Ammoccetes. 

What is the structure of this head-shield ? It has been spoken of 
as formed of bone because it possesses cells, being thus unlike the 
layers of chitin, which are formed by underlying cells but are not 
themselves cellular. At the same time, it is recognized on all sides 
that it has no resemblance to bone-structure as seen in fossil remains 
of higher vertebrates. The latest and best figure of the structure of 
this so-called bone is given in Rohon's paper already referred to. It 
is, so he describes, clearly composed of fibrilkc and star-shaped cells, 
arranged more or less in regular layers, with other sets of similar 
cells and fibrilke arranged at right angles to the first set, or at vary- 
ing angles. The groundwork of this tissue, in which these cells and 
fibrils are embedded, contained calcium salts, and so the whole tissue 
was preserved. In places, spaces are found in it, in the deepest 
layer large medullary spaces; more superficially, ramifying spaces 
which he considers to be vascular, and calls Haversian canals ; the 



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330 THE ORIGIN OF VERTEBRATES 

star-like cells, however, are not arranged concentrically around these 
spaces, as in true Haversian canals. 

This structure is therefore a calcareous infiltration of a tissue 
with cells in it. Where is there anything like it ? 

As soon as I saw Rohon's picture (Fig. 130), I was astounded 
at its startling resemblance to the structure of muco-cartilage as is 
seen in Fig. 131, taken from Ammoccetes. If such muco-cartilage 
were infiltrated with lime salts, then the muco-cartilaginous skeleton 
of Ammoccetes would be preserved in the fossil condition, and be 
comparable with that of Cephalaspis, etc. 







WJfUW WCi'' t\ ?iT«" W V' 



rj(\ 






Fio. 180.— Section of a Head- Fig. 131.— Section of Muco- 

Plate of a Cephalaspid. Cabtilaoe from Dorsal 

(From Rohon.) Head-Plate of Ammoccetes. 

The whole structure is clearly remarkably like Rohon's picture of 
a section of the head-plate of a Cephalaspid (Fig. 130). In the latter 
case the matrix contains calcium salts, in the former it is composed 
of the peculiar homogeneous mucoid tissue which stains so charac- 
teristically with thionin. With respect to this calcification, it is 
instructive to recall the calcification in the interior of the branchial 
cartilages of Limulus, as described in Chapter III., for this example 
shows how easy it is to obtain a calcification in this chondro-mucoid 
material. With respect to the medullary spaces and smaller spaces in 
this tissue, as described by Rohon, I would venture to suggest that they 
need not all necessarily indicate blood-vessels, for similar spaces would 
appear in the head-sliield of Ammocoutes if its muco-cartilage alone 



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RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 33 1 

were preserved. Of these, some would indicate the position of blood- 
vessels, such, for instance, as of the external carotid which traverses 
this structure ; but the largest and most internal spaces, resembling 
Bohon's medullary spaces, would represent muscles, being filled up 
with bundles of the upper lip-muscles. 

The Muco-Cartilaginous Head-Shield of Ammoccetes. 

The resemblance between the structure of the head-shield of 
Thyestes and the muco-cartilage of Ammoccctes, is most valuable, 
for muco-cartilage is unique, occurs in no other vertebrate, and every 
trace of it vanishes at transformation ; it is essentially a character- 
istic of the larval form, and must, therefore, in accordance with all 
that has gone before, be the remnant of an ancestral skeletal tissue. 
The whole story deduced from the study of Ammoccctes would be 
incomplete without some idea of the meaning of this tissue. So 
also, as already mentioned, the skeleton of Ammocoetea is incomplete 
without taking this tissue into account. It is confined entirely to 
the head-region ; no trace of it exists posteriorly to the branchial 
basket-work. It consists essentially of dorsal and ventral head- 
shields, connected together by the tentacular, metastomal, and thyroid 
bars, as already described. The ventral shield forms the muco-carti- 
laginous plate of the lower lip and the plate over the thyroid gland, 
so that the skeleton ventrally is represented by Fig. 118, B, which 
shows how the cartilaginous bars of the branchial basket-work are 
separated from each other by this thyroid plate. At transformation, 
with the disappearance of this muco-cartilaginous plate, the bars 
come together in the middle line, as in the more posterior portion 
of the branchial basket-work. 

The dorsal head-shield of muco-cartilage covers over the upper 
lip, sends a median prolongation over the median pineal eyes and 
a lateral prolongation on each side as far as the auditory capsules, 
giving the shape of the head-shield of muco-cartilage, as in Fig. 
118, C. 

Not only then is the structure of tlje head-shield of a Cephalaspid 
remarkably like the muco-cartilage of Ammocoetes, but also its 
general distribution strangely resembles that of the Ammoco*tes 
muco-cartilage. 

Now, these head-shields in the Cephalaspida? and TremataspidcT 



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332 THE ORIGIN OF VERTEBRATES 

vary very much in shape, as is seen by the comparison of Tre- 
mataspis and Auchenaspis with Cephalaspis and Eukeraspis, and 
yet, undoubtedly, all these forms belong to a single group, the 
Osteostraci. 

The conception that Ammocoetes is the solitary living form allied 
to this group affords a clue to the meaning of this variation of 
shape, which appears to me to be possible, if not indeed probable. 
There is a certain amount of evidence given in the development 
of Ammoccetes which indicates that the branchial region of its 
ancestors was covered with plates of muco-cartilage as well as the 
prosomatic region. 

The evidence is as follows : — 

The somatic muscles of Ammocastes form a continuous longi- 
tudinal sheet of muscles along the length of the body, which are 
divided up by connective tissue bands into a series of imperfect 
segments or myotomes. This simple muscular sheet can be dissected 
off along the whole of the head-region of the animal, with the 
exception of the most anterior part, without interfering with the 
attachments or arrangements of the splanchnic muscular system in 
the least. The reason why this separation can be so easily effected 
is to be found in the fact that the two sets of muscles are not 
attached to the same fascia. The sheet of fascia to which the 
somatic muscles are attached is separated from the fascia which 
encloses the branchial cavity by a space (cf. Figs. 63 and 64) filled 
with blood-spaces and cells containing fat, in which space is also 
situated the cartilaginous branchial basket-work. These branchial 
bars are closely connected with the branchial sheet of fascia, and 
have no connection with the somatic fascia, their perichondrium 
forming part of the former sheet. Upon examination, tliis space 
is seen to be mainly vascular, the blood-spaces being large and 
frequently marked with pigment ; but it also possesses a tissue of its 
own, recognized as fat-tissue by all observers. The peculiarity of 
the cells of this tissue is their arrangement ; they are elongated cells 
arranged at right angles to the plates of fascia, just as the fibres of 
the muco-cartilage are largely arranged at right angles to their 
limiting plates of perichondrium. These cells do not necessarily 
contain fat ; and when they do, the fat is found in the centre of each 
cell, and does not push the protoplasm of the cell to the periphery, 
as in ordinary fat cells. 



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RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 333 



In Fig. 132, B, I give a specimen of this tissue stained by osmic 
acid; in Fig. 132, A, I give a drawing of ordinary muco-cartilage 
taken from the plate of the lower lip; and in Fig. 133, A, a modifi- 
cation of the muco-cartilage taken from the velum, which shows the 
formation of a tissue in- 
termediate between ordi- 
nary muco-cartilage and 
this branchial fat-tissue. 

Further, in fully-grown 
specimens of Ammoccetes, 
in the region of undoubted 
muco-cartilage, a fatty de- 
generation of the cells 
frequently appears, to- 
gether with an increase 
in the blood spaces, — the 
precursor, in fact, of the 




m . pli . . . 



Cop- 



great change which over- 



Dl.br 




takes this tissue soon 
afterwards, at the time of 
transformation, when it is 
invaded by blood, and 
swept away, except in 
those places where new 
cartilage is formed. I 
conclude, then, that the 
tissue of this vascular 
space was originally muco- 
cartilage, which has de- 
generated during the life 
of the Ammoco}tes. The 
fact that in most cases 
undoubted muco-cartilage 

is to be found here and there in this space, is strong confirmation of 
the truth of this conclusion. 

If this conclusion is correct, we may expect that it would be 
confirmed by the embryological history of the tissue, and we ought 
to find that in much younger stages a homogeneous tissue of the 
same nature as muco-cartilage fills up the spaces in the branchial 



Fig. 132. — A, Muco-cartilage op Lower Lip 
(Mc.) ; m.ph., muscle of lower lip ; m.sm., 
somatic muscle ; Cor. t laminated layer of skin. 
B, Degenerated Muco-cartilage of Bran- 
ciiial Region. F., fat layer; P., pigment; 
JH. f blood-space; N. t somatic nerve; tn.br., 
branchial muscle ; m.sm., somatic muscle. 



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334 



THE ORIGIN OF VERTEBRATES 



1 



m 



region, where in the Ammoccetes only blood and fat-containing cells 
are present. For this purpose Shipley kindly allowed me to examine 
his series of sections through the embryo at various ages. These 
specimens are very instructive, especially those stained by osmic 
acid, which preserves the natural thickness of this space better than 
other staining methods. At an age when the branchial cartilages are 
seen to be formed, when no fat -cells are present, a distinctive tissue 
(Fig. 133, B) is plainly visible in the velum and at the base of the 
tentacles, in the very position where in the more advanced Ammo- 
ccetes muco-cartilage exists. Taking, then, this tissue as our guide, 
the specimens show that the space between the skin and the visceral 
muscles in which the cartilaginous basket-work lies is filled with a 

similar material. At this 
stage a sheet of embryonic 
tissue occupies the posi- 
tion where, later on, blood- 
spaces and fat-cells are 
found, and this tissue re- 
sembles that seen in the 
velum and other places 
where muco - cartilage is 
afterwards found. 

I conclude, therefore, 
that originally the bran- 
chial Qr mesosomatic re- 
gion was covered with a 
dorsal plate of muco* cartilage, which carried on its under surface the 
dorsal part of the branchial basket-work, and sprang from the central 
core of skeletogenous tissue around the notochord; this plate was 
separated from the plate which covered this region ventrally by the 
lateral grove in which the gill-slits are situated. The ventral plate 
carried on its under surface the ventral part of the branchial basket- 
work, and was originally continuous with the plate over the thyroid 
gland. 

In Fig. 134, A, B, C, the cranial skeleton of Ammoccetes is 
represented from the dorsal, ventral, and lateral aspects. The 
muco-cartilage is coloured red, the branchial or soft cartilage blue, 
and the hard cartilage purple. The degenerated muco-cartilage of 
e branchial region is represented as an uncoloured plate, on 







Fig. 183. — A, Muco-Cartilage of Velum; 
B, Embryonic Muco-Cartilage of Tentacu- 
lar Bar. 



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RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 335 






Fig. 134.— Skeleton op Head-Region of Ammoccetes. A, Lateral View; B, 
Ventral View; C, Dorsal View. 

Muco-cartilage, red; soft cartilage, blue; hard cartilage, purple, sh^, sk 2 , sk iy 
skeletal bars ; c.e., position of pineal eye ; na. cart., nasal cartilage ; pcd. } pedicle ; 
cr., cranium; nc. , notochord. 



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33 6 THE ORIGIN OF VERTEBRATES 

which the branchial basket-work stands in relief. If it were re- 
stored to its original condition of muco-cartilage, it would represent 
a uniform plate, on the under surface of which the basket-work 
would be situated ; and if it were calcified and made solid, the 
branchial basket-work would not show at all in these figures. 

Is it possible to find the reason why this skeletal covering has 
degenerated so early before transformation, and why the thyroid 
plate remains intact until transformation ? We see that all that part 
which has degenerated is covered over by the somatic muscles, — by, 
in fact, muscles which, being innervated by the foremost spinal 
nerves, belong naturally to the region immediately following the 
branchial. I suggest, therefore, that the original skeletal covering 
of muco-cartilage has remained intact only where it has not been 
invaded and covered over by somatic muscles, but has been invaded 
by blood and undergone the same kind of degenerative change as 
overtakes the great mass of this tissue at transformation wherever 
the somatic muscles have overgrown it. 

The covering somatic muscles in the branchial region form a 
dorsal and ventral group, of which the latter is formed in the embryo 
much later than the former, the line of separation between the two 
groups being the lateral groove, with its row of branchial openings. 
This groove ends at the first branchial opening, but the ventral and 
dorsal somatic muscles continue further headwards. It is instruc- 
tive to see that, although the lateral groove terminates, the separation 
between the two groups of muscles is still marked out by a ridge 
of muco-cartilage, represented in Fig. 134, A, which terminates 
anteriorly in the opercular bar. 

Passing now to the prosomatic region, we find that here, too, the 
muco-cartilaginous external covering is divisible into a dorsal and 
a ventral head-plate, the ventral head-plate being the plate of the 
lower lip, and the dorsal head-plate the plate of muco-cartilage 
over the front part of the head. The staining reaction with thionin 
maps out this dorsal head-plate in a most beautiful manner, and 
shows that the whole of the upper lip-region in front of the nasal 
orifice is one large plate of muco-cartilage, obscured largely by the 
invasion of the crossing muscles of the upper lip, but left pure and 
uninvaded all around the nasal orifice, and where the upper and lower 
lips come together. In addition to this foremost plate, a median 
tongue of muco-cartilage covers over the pineal eye and fills up the 



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RELATIONSHIP OF AMMOCiETES TO OSTRACODERMS 337 

median depression between the two median dorsal somatic muscles. 
Also, two lateral cornua pass caudal wards from the main frontal mass 
of muco-cartilage over the lateral eyes, forming the well-known wedge 
which separates the dorsal and lateral portions of the dorso-lateral 
somatic muscle. In fact, similarly to what we find in the branchial 
region, the muco-cartilaginous covering can be traced with greater 
or less completeness only in those parts which are not covered by 
somatic muscles. 

In Fig. 134, A, B, C, this striking muco-cartilaginous head- 
shield, both dorsal and ventral, is shown. Seeing that the upper lip 
wraps round the lower one on each side, and that this most ventral 
edge of the upper lip contains muco-cartilage, as is seen in Fig. 117, 
the dorsal head-shield of muco-cartilage ought, strictly speaking, to 
extend more ventrally in the drawings. I have curtailed it in order 
not to interfere with the representation of the lower lip and tentacu- 
lar muco-cartilages. 

From what has been said, it follows that the past history of the 
skeletal covering of the whole head-region of Ammoccetes, both 
frontal and occipital, can be conjectured by means of the ontogenetic 
history of the foremost myomeres. 

Dohrn and all other observers are agreed that during the develop- 
ment of this animal a striking forward growth of the foremost somatic 
myomeres takes place, so that, as Dohrn puts it, the body-muscula- 
ture has extended forwards over the gill-region, and at the same 
time the gill-region has extended backwards. It is therefore prob- 
able that in the ancestral form the myotomes, innervated by the first 
spinal nerves, immediately succeeded the branchial region. Judging 
from Ammocci'tes, the forward growth was at first confined to the 
dorsal region, and therefore invaded the dorsal head-plate, the ventral 
musculature being distinctly a later growth. With respect to this 
dorsal part of the myotomes, the first myotome is originally situated 
some distance behind the auditory capsule, and then grows forward 
towards the nasal opening; the lateral part, according to Hatschek, 
grows forward more quickly than the dorsal part, and splits itself 
above and below the eye into a dorso-lateral part, which extends up 
to the olfactory capsule, and a ventro-lateral part (m. lateralis capitis 
anterior, superior, and inferior), thus giving rise to the characteristic 
appearance of the muco-cartilaginous head-shield of Ammocates. 

According, then, to the extent of the growth of these somatic 

z 



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33» 



THE ORIGIN OF VERTEBRATES 



muscles, the shape of the muco-cartilaginous head-shield will vary, 
and if it were calcified and then fossilized we should obtain fossil 
head-shields of widely differing configuration, although such fossils 
might be closely allied to each other. This is just what is found 
in this group. Let the muco-cartilage extend over the whole of 
the branchial region of Ammoccetes, the resulting head-shield would 
be as in Fig. 135, A; the branchial bars below the muco-carti- 
laginous shield might or might not be evident, and the line between 
the branchial and the trigeminal region might or might not be 
indicated. Such a head-shield would closely resemble those of Didy- 
maspis and Tremataspis respectively. Now suppose the somatic 
musculature to encroach slightly on the branchial region and also 




Fig. 136. — Diagrams to show the different shapes of Head-Shields due to 
the forward growth of the somatic musculature. 

A, Didymaspis; B, Auchenaspis ; C, Cephalaspis; P, Ammoccetes. 

laterally to the end of the anterior branchial region, then we should 
obtain a shape resembling that of Thyestes (Fig. 135, B). Continue 
the same process further, the lateral muscle always encroaching 
further than the median masses, until the whole or nearly the whole 
branchial region is invested, and we get the head-shield of Cephalaspis 
(Fig. 135, C) ; further still, that of Keraspis, and yet still further, 
that of Ammocoetes (Fig. 135, D). 

So close is this similarity, from the comparative point of view, 
between the dorsal head-shield of the Osteostraci and the dorsal 
cephalic region of Ammocoetes that it justifies us in taking Ammo- 
ccetes as the nearest living representative of such types ; it is justifi- 
able, therefore, to interpret by means of Ammocoetes the position of 
other organs in these forms. First and foremost is the hard plate 



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RELATIONSHIP OF AMMOCiETES TO OSTRACODERMS 339 

known as the post-orbital plate, so invariably found. In Fig. 134, C, 
I have inserted (cr.) the position of the membranous cranium of 
Ammoccetes, and it is immediately evident that the primordial 
cranium of the Osteostraci must occupy the exact position indicated 
by this median hard plate. For this very reason this median plate 
would be harder than the rest in order to afford a better protection 
to the brain underneath. This plate, because of its position, may 
well receive the same name as the similar plate in the trilobite 
and various palaeostracans and be called the glabellum. 

Evidence of Segmentation in the Head-Shield — Formation 

of Cranium. 

We may thus conceive the position of the nose, lateral eyes, 
median eyes, and cranium in these old fishes. In addition, other 
indications of a segmentation in this head-region have been found. 
The most striking of all the specimens hitherto discovered are some 
of Thyestes ven-ucosm, discovered by Itohon, in which the dorsal 
shield has been removed, and so we are able to see what that dorsal 
shield covered. 

In Fig. 136, I reproduce his drawing of one of his specimens from 
the dorsal and lateral aspects. These drawings show that the frontal 
part of the shield covered a markedly segmented part of the animal ; 
five distinct segments are visible apart from the median most anterior 
region. This segmented region is entirely confined to the prosomatic 
region, i.e. to the region innervated by the trigeminal nerve. An 
indication of similar markings is given in Lankester's figure of 
JEukeraspis pustul if erics (see Fig. 127, B), and, indeed, evidence of 
a segmentation under the antero-lateral border of the head-shield 
is recognized at the present time, not only in the Cephalaspidre, but 
also in the Pteraspidae, as was pointed out to me by Smith Woodward 
in the specimens at the British Museum. Also, in Cyathaspis, Jaekel 
has drawn attention to markings of a similar segmental nature 
(Fig. 137). 

There seems, then, little doubt but that these primitive fishes 
possessed somethiug in this region which was of a segmental character, 
and indicated at least five segments, probably more. 

llohon entitles his discovery ' the segmentation of the primordial 
cranium/ It would, I think, be better to call it the segmentation of 



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340 



THE ORIGIN OF VERTEBRATES 



the anterior region of the head, for that is in reality what his figures 
show, not the segmentation of the primordial cranium, which, to judge 
from Aininocoetes, was confined to the region of the glabellum. 

What is the interpretation of this appearance ? 

Any segmentation in the head-region must be indicative of segments 
belonging to the trigeminal or prosomatic region, or of segments 
belonging to the vagus or mesosomatic region. Many paleontologists, 





Fig. 136.— Lateral and Dorsal Views Fig. 137.— Under Surface of Head- 



op the Frontal and Occipital Regions 
of the Head-Shield of Thyestes. after 
Removal of the Outer Surface. (From 
Rohon.) 



Shield of Cyathaspis. (From 
Jaekel.) 

^1., lateral eyes ; Ejk, median eyes. 



looking upon segmentation as indicative of gills and gill-slits, have 
attempted to interpret such markings as branchial segments, regard- 
less of their position. As the figures show, they extend in front of 
the eyes and reach round to the front middle line, a position which 
is simply impossible for gills, but points directly to a segmentation 
connected with the trigeminal nerve. Comparison with Ammocutes 
makes it plain enough that the markings in question are prosomatic 
in position, and that the gill-region must be sought for in the place 



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RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 34 1 



where Schmidt and Eohon located it in Thyestes, viz. the so-called 
occipital region. 

This discovery of Bohon's is, in my opinion, of immense importance, 
for it indicates that, in these early fishes, the prosomatic segmenta- 
tion, associated with the trigeminal nerve, was much more well- 
marked than in any fishes living in the present day. Why should 
it be more well-marked? Turning to the paheostracan, it is very 
suggestive to compare the markings on their prosomatic carapace 
with these markings. Again and again we find indications of seg- 
mentation in these fossils similar to those seen in the ancient fishes. 
Thus in Fig. 138 I have put side by side the pala^ostracan Bunodes 
and the fish Thyestes, both life 
size. In the latter I have indicated 
Ilohon's segments ; in the former the 
markings usually seen. 

From the evidence of Phrynus, 
My gale, etc., as already pointed out, 
such markings in the pala^ostracan 
fossils would indicate the position of 
the tergo-coxal muscles of the pro- 
somatic appendages, even though 
such appendages have not yet been 
discovered, and it is significant that 
in all these cases there is a distinct 
indication of a median plate or 
glabellum in addition to the seg- 
mental markings. Especially instructive is the evidence of Phrynus, 
as is seen by a comparison of Figs. 107 and 108, which shows clearly 
that this median plate (glah) covered the brain-region, a brain-region 
which is isolated and protected from the tergo-coxal muscles by the 
growth dorsalwards of the flanges of the plastron. In this way an 
incipient cranium of a membranous character is formed, which helps 
to give attachment to these tergo-coxal muscles. As such cranium 
is derived directly from the plastron, it is natural that it should 
ultimately become cartilaginous, just as occurs when Ammoca»tes 
becomes Petromyzon and the cartilaginous cranium of the latter 
arises from the membranous cranium of the former. In Galeodes 
also the growth dorsalwards of the lateral flanges of the plastron to 
form an incipient cranium in which the brain lies is very apparent. 




A 

Fig. 138.— A, Outline op Thyestes 
Verrucosus with Bohon's Seg- 
ments indicated ; B, Outline op 
Bunodes Lunula with Lateral 
Eyes inserted. 

Both figures natural size. 



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342 THE ORIGIN OF VERTEBRATES 

I venture, then, to suggest that in the Osteostraci the median 
hard plate or glabellum protected a brain which was enclosed in a 
membranous cranium, very probably not yet complete in the dorsal 
region — certainly not complete if the median pineal eyes so univer- 
sally found in these ancient fishes were functional — a cranium derived 
from the basal trabecule, in precisely the same manner as we see it 
already in its commencement in Phrynus and other scorpions. With 
the completion of this cranium and its conversion into cartilage, and 
subsequently into bone, an efficient protection was afforded to the 
most vital part of the animal, and thus the hard head-shield of the 
Paltf03traca and of the earliest fishes was gradually supplanted by 
the protecting bony cranium of the higher vertebrates. 

Step by step it is easy to follow in the mind's eye the evolution 
of the vertebrate cranium, and because it was evolved direct from 
the plastron, the impossibility of resolving it into segments is at 
once manifest ; for although the plastron was probably originally 
segmented, as Schimkewitsch thinks, all sign of such segmentation 
had in all probability ceased, before ever the vertebrates first made 
their appearance on the earth. 

It follows further, from the comparison here made, that those 
antero-lateral markings indicative of segments, found so frequently 
in these primitive fishes, must be interpreted as due not to gills but 
to aponeuroses, due to the presence of muscles which moved proso- 
matic appendages, muscles which arose from the dorsal region in 
very much the same position as do the muscles of the lower lip in 
Ammocoetes ; the latter, as already argued, represent the tergo-coxal 
muscles of the last pair of prosomatic appendages — the chilaria or 
metastoma. Such an interpretation of these markings signifies that 
the first- formed fishes must have possessed prosomatic appendages of 
a more definite character than the tentacles of Ammocoetes, something 
intermediate between those of the paheostracau and Ammocoetes. 

For my part I should not be in the least surprised were I to hear 
that something of the nature of appendages in this region had been 
found, especially in view of the well-known existence of the pair of 
appendages in the members of the Asterolepidse — large, oar-like 
appendages which may well represent the ectognaths. 



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RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 343 

The Relationship of the Ostracoderms. 

Of the three groups of fishes — the Heterostraci, the Osteostraci, 
and the Antiarcha — the last is Devonian, and therefore the latest 
in time of the three, while the earliest is the first group, as both 
Pteraspis and Cyathaspis have been found in lower levels of the 
Silurian age than any of the Osteostraci, and, indeed, Cyathaspis 
has been discovered in Sweden in the lower Silurian. This, the 
earliest of all groups of fishes, is confined to two forms only — 
Pteraspis and Cyathaspis, — for Scaphaspis is now recognized to be 
the ventral shield of Pteraspis. 

Hitherto a strong tendency has existed in the minds both of the 
comparative anatomist and the paleontologist to look on the elasmo- 
branchs as the earliest fishes, and to force, therefore, these strange 
forms of fish into the elasmobranch ranks. For this purpose the 
same device is often used as has been utilized in order to account 
for the existence of the Cyclostomata, viz. that of degeneration. The 
evidence I have put forward is very strongly in favour of a con- 
nection between the cyclostomes and the cephalaspids, and agrees 
therefore with all the rest of the evidence that the jawless fishes 
are more ancient than those which bore jaws — the Gnathostomata. 

This is no new view. It was urged by Cope, who classified the 
Heterostraci, Osteostraci, and Antiarcha under one big group — the 
Agnatha — from which subsequently the Gnathostomata arose. Cope's 
arguments have not prevailed up to the present time, as is seen in 
the writings of Traquair, one of the chief authorities on the subject 
in Great Britain. He is still an advocate of the elasmobranch origin 
of all these earliest fishes, and claims that the latest discoveries of 
the Silurian deposits (Thelodm Pagri) and other meml>ers of the 
Ccelolepidae confirm this view of the question. 

This view may be summed up somewhat as follows : — 

Cartilaginous jaws would not fossilize, and the Ostracoderms may 
have possessed them. 

They may have degenerated from elasmobranchs just as the 
cyclostomes are supposed to have degenerated. 

Seeing that bone succeeds cartilage, the presence of bony shields 
in Cephalaspis, etc., indicates that their precursors were cartilaginous, 
presumably elasmobranch fishes. 

Of these arguments the strongest is based on the supposed bony 



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344 THE ORIGIN OF VERTEBRATES 

covering of the Osteostraci, with the consequent supposition that 
their ancestors possessed a cartilaginous covering This argument is 
entirely upset, if, as I have pointed out, the structure of the cepha- 
laspid shield is that of muco-cartilage and not of bone. If these 
plates are a calcified muco-cartilage, then the whole argument for 
their ancestry from animals with a cartilaginous skeleton falls to the 
ground, for muco-cartilage is the precursor not only of bone, but also 
of cartilage itself. 

The evidence, then, points strongly in favour of Cope's view that 
the most primitive fishes were Agnatha, after the fashion of cyclo- 
stomes, as is also believed by Smith Woodward, Bashford Dean, and 
Jaekel. 

Among living animals, as I have shown, the Limulus is the sole 
survivor of the paheostracan type, and Ammoccetes alone gives a 
clue to the nature of the cephalaspid, i.e. the osteostracan fish. Older 
than the latter is the heterostracan, Pteraspis, and Cyathaspis. Is 
it possible from their structure to obtain any clue as to the actual 
passage from the pakeostracan to the vertebrate ? 

Here again, as in the case of the Osteostraci, a relationship to the 
elasmobranch has been supposed, for the following reasons : — 

The latest discoveries in the Silurian and Devonian deposits have 
brought to light strange forms such as Thelodus and Drepanaspis, of 
which the latter from the Devonian must, according to Traquair, be 
included in the Heterostraci. It possessed, as seen in Fig. 139, large 
plates, after the fashion of Pteraspis, and also many smaller ones. 

The former, from the upper Silurian, belongs to the GVvlolepidi^, 
and was covered over with shagreen composed of small scutes, after 
the fashion of an elasmobranch. Traquair suggests that Thelodus 
arose from the original elasmobranch stock ; that by the fusion of 
scutes such a form as Drepanaspis occurred, and, with still further 
fusion, Pteraspis. 

There are always two ways of looking at a question, and it seems 
to me possible and more probable to turn the matter round and to 
argue that the original condition of the surface- covering was that of 
large plates, as in Pteraspis. By the subsequent splitting up of such 
plates, Drepanaspis was formed, and later on, by further splitting, 
the elasmobranch, Thelodus being a stage on the way to the forma- 
tion of an elasmobranch, and not a backward stage from the elasmo- 
branch towards Pteraspis. 



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RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 345 

This method of looking at the problem seems to me to be more 
in consonance with the facts than the reverse ; for, as pointed out by 
Jaekel, the fishes with large plates are the oldest, and in Cyathaspis, 
the very oldest of all, the size of the plates is most conspicuous; he 
considers, therefore, this preconceived view that large plates are 
formed by the fusion of small ones must give way to the opposite 
belief. 

So also llohon, as quoted by Traquair, who, in his first paper 
accepted Lankester's view that the ridges of the pteraspidian shield 





Fio. 139.— Drepanaspis. Ventral and Dorsal Aspects. (After Lankestep.) 
A., anus; E., lateral eyes. 

were formed by the fusion of a linear arrangement of numbers of 
placoid scales, suggests in his second paper that these ridges may 
have been the most primitive condition of the dermal skeleton of the 
vertebrate, out of which, by differentiation, the dermal denticles 
(placoid scales) of the selachian, as well as their modifications in the 
ganoids, teleosteans, and amphibians, have arisen. 

One thing is agreed upon on all sides ; no sign of bone-corpuscles 
is to be found in this dermal covering of Fteraspis. In the deeper 
layers are large spaces, the so-called pulp-cavities leading into 
naiTow canaliculi, the so-called dentine canals. The structure is 



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346 THE ORIGIN OF VERTEBRATES 

looked upon as similar to that of the pulp and dentine canals of 
many fish- scales. 

On the other hand, this dermal covering of Pteraspis has been 
compared by Patten with the arrangement of the chitinous structure 
of certain parts of the external covering of Limulus, a comparison 
which to my mind presents a great difficulty. The chitin-layers in 
Limulus are eo:ternal to the epidermal cells, being formed by them; 
the layers in Pteraspis which look like chitin must have been internal 
to the epidermal layer; for each vascular canal which passes from 
a pulp-cavity on its way to be distributed into the dentine canals 
of the ridge gives off short side branches, which open directly 
into the groove between the ridges. If these canals were filled with 
blood they could not possibly open directly into the open grooves 
between the ridges ; these openings must, therefore, have been covered 
over with an epithelial layer which covered over the surface of the 
animal, and consequently the chitin-like structure must have been 
internal to the epidermis, and not external, as on Patten's view. 
The comparison of this structure with the dentine of fish -scales 
signifies the same tiling, for in the latter the epidermis is external 
to the dentine-plates, the hard skeletal structure is in the position 
of the cutis, not of the cuticle. 

The position appears to me to be this : the dermal cranial skele- 
ton of vertebrates, whether it takes the form of a bony skull or of 
the dorsal plates of a cephalaspid or a pteraspid is, in all cases, not 
cuticular, i.e. is not an external formation of the epidermal cells, but 
is formed in tissue of the nature of connective tissue underlying the 
epidermis. On the contrary, the hard part of the head-carapace of 
the palaeostracan is an external formation of the epidermal cells. 

If, then, this tissue of Pteraspis is not to bo looked upon as 
chitin, how can we imagine its formation ? It is certainly not bone, 
for there are no bone-corpuscles; it is a very regular laminated 
structure resembling in appearance chitin rather than anything else. 

As in all cases of difficulty, turn to Ammocotes and let us see 
what clue there is to be found there. The skin of Ammocoetes is 
peculiar among vertebrates in many respects. It consists of a number 
of epidermal cells, as in Fig. 140, the varying function of which 
need not be considered here, covered over with a cuticular layer 
which is extraordinarily thick for the cuticle of a vertebrate skin ; this 
cuticular layer is perforated with fine canaliculi, through which the 



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RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 347 



secretion of the underlying cells passes, as is seen in Fig. 140, A and 
B. This cuticle corresponds to the chitinous covering of the 
arthropod, and like it is perforated with canaliculi, and, according to 
Lwoff, possibly contains chitin. The epidermal cells rest on a thick 
layer of most striking appearance (Fig. 141), for it resembles, in an 
extraordinary degree, when examined superficially, a layer of chitin ; 
it is called the laminated layer, and is characterized by the extreme 
regularity of the laminae. This appearance is due, as the observa- 
tions of Miss Alcock show, to alternate layers of connective tissue 
fibres arranged at right angles to each other, each fibre running a 
straight course and possessing its own nucleus. Although the fibres 
in each layer are packed close together, they are sufficiently apart 
to form with the fibres of the 
alternate layers a meshwork 
rather than a homogeneous 
structure, and thus the surface 
view of this layer shows a 
regular network of very fine 
spaces through which nerve- 
fibres and fluid pass. This 
layer is easily dissolved in a 
solution of hypochlorite of soda, 
a fluid which dissolves chitin. 
Any one looking at Ammo- 
coetes would say that the only 
part of its skin which resembles 

chitin is this laminated layer, and therefore the only part of its 
skin which would afford an indication of the nature of the 
skeleton of Pteraspis is this laminated layer, which belongs to 
the cutis, and not to the cuticle. Yet another significant peculiarity 
of this layer is its entire disappearance at transformation. Miss 
Alcock, in a research not yet published, has shown that this layer 
is completely broken up and absorbed at transformation ; the cutis 
of Petromyzon is formed entirely anew, and no longer presents any 
regular laminated character, but resembles rather the sub-epidermal 
connective tissue layer of the skin of higher vertebrates. This 
laminated layer, then, just like the muco-cartilage, shows, by its 
complete disappearance at transformation, its ancestral character. 
Very suggestive is the arrangement of the; different skeletal 





Fig. 140.— Epithelial Cells op Ammo- 
ccetes to show the canaliculi in the 
Thick Cuticle (B). A, Transverse 
Section through the Cuticle. 



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34« 



THE ORIGIN OF VERTEBRATES 







tissues in the head-region of Amnioccetes. Fig. 141 represents a 
section through the head near the pineal eye. Most internally is a, 
a section of the membranous cranium, then comes &, the muco- 
cartilaginous skeleton, then c, the laminated layer, and finally d, the 
external cuticle. If in Ammoccetes we possess an epitome of the 
history of the vertebrate, how would these layers be represented in 

the past ages, supposing they 
could be fossilized ? 

The most internal layer a, by 
the formation of cartilage and 
then bone, represents the great 
mass of vertebrate fossils ; the 
next layer b, by a process of 
calcification, as previously argued, 
represents the head-shield of the 
Osteostracan fishes; while the 
cuticular layer d, no longer thin, 
is the remnant of the Palaeo- 
stracan head-carapace. Between 
these two layers, b and d, lies the 
laminated layer c. Intermediate 
to the Paheostracan and the Osteo- 
stracan comes the Heterostracan, 
with its peculiar head-shield — a 
head-shield whose origin is more 
easily conceivable as arising from 
something of the nature of the 
laminated layer than from any 
other structure represented in 
Ammoccotes. 

My present suggestion, then, 
is this : the transition from the 
skeletal covering of the Pala^ostracan to that of the highest verte- 
brates was brought about by the calcification of successive layers 
from without inwards, all of which still remain in Ammoca*tes and 
show how the external chitinous covering of the arthropod was 
gradually replaced by the deep-lying internal bony cranium of the 
higher vertebrates. 

In Ammoco'tes the layer which represents the covering of the 



mm 



Mm., 




Fig. 141.— Section of Skin and Under- 
lying Tissues in the Head-Region 
of Ammoccetes. 

a, cranial wall ; 6, muco-cartilagc ; c, 
laminated layer ; d, external cuticular 
layer. 



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RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 349 

Palteostracan has already almost disappeared. At transformation 
the layers representing the stage arrived at by the Heterostracan 
and the Osteostracan disappear ; but the stage representing the 
higher vertebrates, far from disappearing, by the formation of carti- 
lage reaches a higher stage and prepares the way for the ultimate 
stage of all — the formation of the bony cranium. 

So much for the evidence as to the nature of the structure of the 
head-shield of the Pteraspidae. 

It suggests that these fishes were covered anteriorly with armoured 
plates derived from the cutis layer of the skin, a layer which was 
specially thickened and very vascular, apparently, to enable respi- 
ration to be very largely, if not entirely, effected by the surface 
of the body. It is difficult to understand how the sea-scorpions 
breathed, and it is easy to see how the formation of ventral and 
dorsal plates enclosing the mesosomatic appendages may at the outset 
have hindered the action of the branchiae. The respiratory chamber, 
according to my view, had at first the double function of respiration 
and digestion. A new digestive apparatus was the pressing need at 
the time ; it would, therefore, be of distinct advantage to remove, as 
much as possible, the burden of respiration from this incipient 
alimentary canal. 

What can be said as to the shape of these ancient forms of 
fishes ? Certain parts of them are absolutely known, other parts are 
guesswork. They are known to have possessed a dorsal shield, a 
ventral shield formerly looked upon as belonging to a separate species, 
called Scaphaspis, and a spine attached to the dorsal shield. The 
rest of their configuration, as given in Smith Woodward's restoration 
(Fig. 142) is guesswork ; the fish-like body with its scales, the hetero- 
cercal tail, is based on the most insufficient evidence of something 
of the nature of scales having being found near the head-plates. 

The dorsal shield is characterized by a pair of lateral eyes 
situated on the edge of the shield, not as in Cephalaspis near the 
middle line. In the middle line, where the rostrum meets the large 
dorsal plate, median eyes were situated. But the slightest sign of 
any median single nasal opening, such as is so characteristic of the 
head-shield of the Osteostraci and of Ammoctetes has never been 
discovered. The olfactory organ must have been situated on the 
ventral side as in the larval stage of Ammocates, or in the Faheo- 
straca. Many of these head-shields are remarkably well preserved, 



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55o 



THE ORIGIN OF VERTEBRATES 



and it is difficult to believe that an olfactory opening would not be 
seen if any such had existed, as it does in Thyestes. 

The difficulty of interpreting these types is the difficulty of under- 
standing their method of locomotion ; that is largely the reason why 
the spine has been placed as if projecting from the back, and a fish- 
like body with a heterocercal tail-fin added. If, on the contrary, the 





Fig. 142.— Restobation of Ptebaspis. (After Smith Woodwabd.) 

spine is a terminal tail-spine, then, as far as the fossilized remains 
indicate, the animal consisted of a dorsal shield, a ventral shield, and 
a tail-spine, to which must be added two apparently lateral pieces 
and a few scales. If the animal did not possess a flexible body with 
a tail-fin, but terminated in a rigid spike after the fashion of a 
Liinulus-like animal, then it must have moved by means of 



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RELATIONSHIP OF AMAIOCCETES TO OSTRACODERMS 35 1 

appendages. At present we have not sufficient evidence to decide 
this question. 

That the animal crawled about in the mud by means of free 
appendages is by no means an impossible view, seeing how difficult 
it is to find the remains of appendages in the fossils of this far-back 
time, even when we are sure that they existed. Thus, for many 
generations, the appendages of trilobites, which occur in such count 
less numbers, and in such great variety of form, were absolutely 
unknown, until at last, in consequence of a fortunate infiltration 
by pyrites, they were found by Beecher preserved down to the 
minutest detail. Even to this day no trace of appendages has been 
found in such forms as Hemiaspis, Bunodes, Belinurus, Prestwichia. 

The whole question of the evidence of any prosomatic appendages 
in these ancient fishes is one of very great interest, and of late years 
has been investigated by Patten. It has long been known that 
forms such as Pterichthys and Bothriolepis possessed two large, jointed 
locomotor appendages, and Patten has lately obtained better speci- 
mens of Bothriolepis than have ever been found before, which show 
not only the general configuration of the fish, but also the presence 
of mandibles or gnathites in the mouth-region resembling those of 
an arthropod. These mandibles had been seen before (Smith Wood- 
ward), but Patten's specimens are more perfect than any previously 
described, and cause him to conclude that these ancient fish were 
of the nature of arthropods rather than of vertebrates. 

Patten has also been able to obtain some excellent specimens of 
the under surface of the head of Tremataspis, which, as evident in 
Fig. 143, show the presence of a series of holes, ranging on each side 
from the mouth-opening, in a semicircular fashion towards the middle 
line. He considers that these openings indicate the attachments of 
appendages, in opposition to other observers, such as Jaekel, who look 
upon them as gill-slits. To my mind, they are not in the right 
position for gill-slits ; they are certainly in a prosomatic rather than 
in a mesosomatic position, and I should not be at all surprised if 
further research justified Patten's position. So convinced is he of 
the presence of appendages in all these old forms, that he considers 
them to be arthropods rather than vertebrates, although, at the same 
time, he looks upon them as indicating the origin of vertebrates from 
arthropods. Here, perhaps, it is advisable to say a few words on 
Patten's attitude towards this question. 



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352 



THE ORIGIN OF VERTEBRATES 



Two years after I had put forward my theory of the derivation 
of vertebrates from arthropods, Patten published, in the Quarterly 
Journal of Microscopical Science, simultaneously with my paper in 
that journal, a paper entitled "The Origin of Vertebrates from 
Arachnids." In this paper he made no reference to my former 
publications, but he made it clear that there was an absolutely 
fundamental difference between our treatment of the problem; for 
he took the old view that of necessity there must be a reversal of 

surfaces in order that the 
internal organs should be 
in the same relative positions 
in the vertebrate and in the 
invertebrate. He simply, 
therefore, substituted Arach- 
nid for Annelid in the old 
theoiy. Because of this 
necessity for the reversal 
of surfaces he discarded the 
terms dorsal and ventral as 
indicative of the surfaces of 
an animal, and substituted 
haemal and neural, thereby 
hopelessly confusing the 
issue and making it often 
very difficult to understand 
his meaning. 

He still holds to his 
original opinion, and I am 
still waiting to find out 
when the reversal of sur- 
faces took place, for Ids investigations lead him, as must naturally 
be the case, to compare the dorsal (or, as he would call it, the 
hiemal) surface of Bothriolepis, of the Cephalaspidie, and of the 
Pteraspidie with the dorsal surface of the Palteostraca. 

All these ancient fishes are, according to him, still in the arthro- 
pod stage, have not yet turned over, though in a peculiarly unscien- 
tific manner lie argues elaborately that they must have swum on 
their back rather than on their front, and so indicated the coming 
reversal. Because they were arthropods they cannot have had a 




Fig. 148.— Undeb-Surface of Head-Kegion 
in Tremataspis. (After Patten.) 



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RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 353 

frontal nose-organ; therefore, Patten looks upon the nose and the 
two lateral eyes of the Osteostraci as a complex median eye, regard- 
less of the fact that the median eyes already existed. 

Every atom of evidence Patten has brought forward, every new 
fact he has discovered, confirms my position and makes his still more 
hopelessly confused. Keep the animal the right side uppermost, and 
the evidence of the rocks confirms the transition from the Palaeo- 
stracan to the Cyclostome ; reverse the surfaces, and the attempt to 
derive the vertebrate from the palaeostracan becomes so confused and 
hopelessly muddled as to throw discredit on any theory of the origin 
of vertebrates from arthropods. For my own part, I fully expect 
that appendages will be found not only in the Cephalaspid® but also 
in the Pteraspidse, and I hope Patten will continue his researches 
with increasing success. I feel sure, however, his task will be much 
simplified if he abandons his present position and views the question 
from my standpoint. 

Summary. 

The shifting" of the nasal tube from a ventral to a dorsal position, as seen 
in Ammocoetes, is, perhaps, the most important of all clues in connection with 
the comparison of Ammocoetes to the PalsBostracan on the one hand, and to the 
Cephalaspid on the other ; for, whereas the exact counterpart of the opening 
of such a tube is always found on the dorsal head-shield in all members of the 
latter gTOup, nothing of the kind is ever found on the dorsal carapace of the 
former group. 

The reason for this difference is made immediately evident in the develop- 
ment of Ammocoetes itself, for the olfactory tube originates as a ventral tube — 
the tube of the hypophysis— in exactly the same position as the olfactory tube of 
the Palaeostracan, and later on in its development takes up a dorsal position. 

In fact, Ammocoetes in its development indicates how the Palieostracan 
head-shield became transformed into that of the Cephalaspid. 

In another most important character Ammocoetes indicates its relationship 
to the Cephalaspidee, for it possesses an external skeleton or head-shield composed 
of muco-cartilage, which is the exact counterpart of the so-called bony head- 
shield of the latter group; and still more strikingly the structure of the 
cephalaspidian head-shield is remarkably like that of muco-cartilage. In the one 
case, by the deposition of calcium salts, a hard external skeleton, capable of 
being preserved as a fossil, has been formed ; in the other, by the absence of 
the calcium salts, a soft chondro-mucoid matrix, in which the characteristic 
cells and fibrils are embedded, distinguishes the tissue. 

The recognition that the head-shields of these most primitive fishes were 
not composed of bone, but of muco-cartilage, the precursor of both cartilage 
and bone, immediately clears up in the most satisfactory manner the whole 

2 A 



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354 THE ORIGIN OF VERTEBRATES 

question of their derivation from elasmobranch fishes ; for the main argument in 
favour of the latter derivation is the exceedingly strong one that bone succeeds 
cartilage— not vice versa — therefore, these forms, since their head-shield is bony, 
must have arisen from some other fishes with a cartilaginous skeleton, most 
probably of an elasmobranch nature. Seeing, however, that the structure of 
their shields resembles muco-cartilage much more closely than bone, and that 
Ammoccetes forms a head-shield of muco-cartilage closely resembling theirs, 
there is no longer any necessity to derive the jawless fishes from the gnatho- 
stomatous ; but, on the contrary, we may look with certainty upon the Agnatha 
as the most primitive group from which the others have been derived. 

The history of the rocks shows that the group of fishes, Pteraspis and 
Cyathaspis, are older than the Cephalaspidte— come, therefore, phylogenetically 
between the Palceostraca and the latter group. In this group the head- 
shields are of a very different character, without any sign of any structure 
comparable with that of bone, and although they possessed both lateral and 
median eyes, there is never in any case any trace of a dorsal nasal orifice. 
Their olfactory passage, like that of the Palaeostraca, must have been ventral. 

The remarkable comparison which exists between the head-shields of 
Ammoccetes and Cephalaspis, enables us to locate the position of the brain and 
cranium of the latter with considerable accuracy, and so to compare the 
segmental markings found in many of these fossils with the corresponding 
markings, found either in fossil Palaeostraca or on the head-carapaces of living 
scorpions and spiders, such as Phrynus and Mygale. In all cases the cranial 
region was covered with a median plate, often especially hard, which corre- 
sponded to the glabellum of the trilobite ; the growth of the cranium can be 
traced from its beginnings' as the upturned lateral flanges of the plastron to the 
membranous cranium of AmmocoBtes. 

From such a comparison it follows that the segments, found in the antero- 
lateral region of the head-shield, were not segments of the cranium, but of parts 
beyond the region of the cranium, and from their position must have been 
segments supplied by the trigeminal nerve, and not by the vagus group; 
segments, therefore, which did not indicate gills and gill-slits, but muscles, 
innervated by the trigeminal nerve ; muscles which, as indicated by the corre- 
sponding markings on the carapace of Phrynus, Mygale, etc., were the tergo- 
coxal muscles of the prosomatic appendages. 

The discovery of the nature of these appendages in the Pteraspidae 
and Cephalaspidre. as well as in the Asterolepidaa (Pterichthys and Bothrio- 
lepis), is a problem of the future, though in the latter, not only have the 
well-known oar-like appendages been long since discovered, but Patten has 
recently found specimens of Bothriolepis which throw light on the anterior 
masticating gnathite-like appendages which these ancient forms possessed. 



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CHAFTER XI 

THE EVIDENCE OF THE AUDITORY APPARATUS AND 
THE ORGANS OF THE LATERAL LINE 

Lateral line organs. — Function of this group of organs. — Poriferous sense- 
organs on the appendages in Limulus. — Branchial sense-organs. — Proso- 
matic sense organs. — Flabellum. — Its structure and position.— Sense-organs 
of mandibles. — Auditory organs of insects and arachnids. — Poriferous 
chordotonal organs. — Balancers of Diptera. — Resemblance to organs of 
flabellum. — Racquet-organs of Galeodes.— Pectens of scorpions. — Large 
size of nerve to all these special sense-organs. — Origin of parachordals and 
auditory capsule. — Reason why Vllth nerve passes in and out of capsule. — 
Evidence of Ammocoetes. — Intrusion of glandular mass round brain into 
auditory capsule. — Intrusion of generative and hepatic mass round brain 
into base of flabellum. — Summary. 

When speaking of the tripartite arrangement of the cranial nerves, 
an arrangement which gave the clue to the meaning of the cranial 
segments, I spoke of the trigeminal as supplying the sensory nerves 
to the skin in the head-region, and I compared this dorsal system 
of afferent nerves to the system of epimeral nerves in Limulus which 
supply the prosomatic and mesosornatic carapaces of Limulus with 
sensory fibres. I compared the ventral system of eye-muscle nerves 
with the system of nerves supplying the segmental dorso-ventral 
somatic muscles of the prosomatic region, and I compared the lateral 
system of mixed nerves with the nerves supplying the prosomatic 
and mesosornatic appendages of Limulus. I compared, also, the 
optic nerves and the olfactory nerves with the corresponding nerves 
in the same invertebrate group. My readers will see at once that one 
well-marked group of nerves — the auditory aud lateral line system — 
has been entirely omitted up to the present, it has not even been 
mentioned in the scheme of the cranial segments ; I have purposely 
reserved its consideration until now, because the organs these nerves 
supply, though situated in the skin, are of such a special character 



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356 THE ORIGIN OF VERTEBRATES 

as to form a category by themselves. These nerves cannot be classed 
among the afferent nerves of the skin any more than the nerves of the 
optic and olfactory apparatus; they require separate consideration. 
A very extensive literature has grown up on the subject of this 
system of lateral line sense-organs and their innervation, the outcome 
of which is decisively in favour of this system being classed with the 
sense-organs supplied by the auditory nerve, so that in endeavouring 
to understand the position of the auditory nerve, we must always 
bear in mind that any theory as to its origin must apply to the 
system of lateral line nerves as well. 

Now, although the auditory apparatus is common to all verte- 
brates, the lateral line system is not found in any land-dwelling 
animals; it belongs essentially to the fishes, and is, therefore, an 
old system so far as concerns the vertebrate group. Its sense-organs 
are arranged along the lateral line of the fish, and, in addition, on 
the head-region in three well-marked lines known as -the supra- 
orbital, infra-orbital, and mandibular line systems. These sense- 
organs lie in the skin in a system of canals, and are innervated by 
a special nervous system different to that innervating adjacent skin- 
areas. The great peculiarity of their innervation consists in the fact 
that their nerves all belong to the branchial system of nerves ; no 
fibres arise in connection with the trigeminal, but all of them in 
connection with the facial, glossopharyngeal and vagus nerves. In 
other words, although organs in the skin, their nerve-supply belongs 
to the lateral nervous system which supplies splanchnic and not 
somatic segments, a system which, according to the theory advanced 
in this book, originated in the nerves supplying appendages. The 
conclusion, therefore, is that in order to obtain some clue as to the 
origin of the sense-organs of this system in the assumed palaeostracan 
ancestor, we must examine the mesosomatic appendages and see 
whether they possess any special sense-organs of similar function. 

Further, considering that the auditory organ is to be regarded 
as a specially developed member of this system, we must especially 
look for an exceptionally developed organ in the region supplied 
by the auditory nerve. 

The question of the origin of this system of lateral line sense- 
organs possesses a special interest for all those who attempt to obtain 
a solution of the origin of vertebrates, for the upholders of the view 
that the vertebrates have descended from annelids have always 



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THE EVIDENCE OF THE AUDITORY APPARATUS 357 

found its strongest support in the similarity of two sets of segmental 
organs found in annelids and vertebrates. On the one hand, great 
stress Was laid upon the similarity of the segmental excretory organs 
in the two groups of animals, as will be discussed later ; on the other, 
of the similarity of the segmentally arranged lateral sense-organs. 

These lateral sense-organs of the annelids have been specially de- 
scribed by Eisig in the Capitellidte, and, according to Lang, "there are 
many reasons for considering these lateral organs to be homologous 
with the dorsal cirri of the ventral parapodia of other Polychieta, aud 
in the family of the Glyceriche we can follow, almost step by step, 
the transformation of the cirri into lateral organs." Eisig describes 
them in the thoracic prebranchial region as slightly different from 
those in the abdominal branchial region ; in the latter region, the 
ventral parapodia are gill-bearing, so that these lateral organs are 
in the branchial region closely connected with the branchiie, just 
as is also the case in the vertebrates. It is but a small step from 
the gill-bearing ventral parapodia of the annelid to the gill-bearing 
appendages of the phyllopod-like protostracau ; so that if we assume 
that this is the correct line along which to search for the origin of 
the vertebrate auditory apparatus, then, on my theory of the origin 
of the vertebrates from a group resembling the Protostraca, it follows 
that special sense-organs must have existed either on or in close 
connection with the branchial and prebranchial appendages of the 
protostracan ancestor of the vertebrates, which would form an inter- 
mediate link between the lateral organs of the annelids and the 
lateral and auditory organs of the vertebrates. 

Further, these special sense-organs could not have l>een mere 
tactile hairs, but must have possessed some special function, and 
their structure must have been compatible with that function. Can 
we obtain any clear conception of the original function of this whole 
system of sense-organs ? 

A large amount of experimental work has been done to determine 
the function of the lateral line organs in fishes, and they have been 
thought at one time or another to be supplementary organs for 
equilibration, organs for estimating pressure, etc. The latest experi- 
mental work done by Parker points directly to their l>eing organs 
for estimating slow vibrations in water in contradistinction to the 
quicker vibrations constituting sound. He concludes that surface 
wave-movements, whether produced by air moving on the water or 



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358 THE ORIGIN OF VERTEBRATES 

solid bodies falling into the water, are accompanied by disturbances 
which are stimuli for the lateral line organs. 

One of these segmental organs has become especially important 
and exists throughout the whole vertebrate group, whether the animal 
lives on land or in water — this is the auditory organ. Throughout, 
the auditory organ has a double function — the function of hearing 
and the function of equilibration. If, then, this is, as is generally 
supposed, a specialized member of the group, it follows that the 
less specialized members must possess the commencement of both 
these functions, just as the experimental evidence suggests. 

In our search, then, for the origin of the auditory organ of verte- 
brates, we must look for special organs for the estimation of vibra- 
tions and for the maintenance of the equilibrium of the animal, 
situated on the appendages, especially the branchial or mesosomatic 
appendages ; and, further, we must specially look for an exceptional 
development of such segmental organs at the junction of the pro- 
somatic and mesosomatic regions. 

Throughout this book the evidence which I have put forward 
has in all cases pointed to the same conclusion, viz. that the verte- 
brate arose by way of the Cephalaspidae from some arthropod, either 
belonging to, or closely allied to, the group called Palseostraca, of 
which the only living representative is Limulus. If, then, my argu- 
ment so far is sound, the appendages of Limulus, both prosomatic 
and mesosomatic, ought to possess special sense-organs which are 
concerned in equilibration or the appreciation of the depth of the 
water, or in some modification of such function, and among these 
we might expect to find that somewhere at the junction of the pro- 
soma and mesosoma such sense-organs were specially developed to 
form the beginning of the auditory organ. 

Now, it is a striking fact that the appendages of Limulus do 
possess special sense-organs of a remarkable character, which are 
clearly not simply tactile. Thus Gegenbaur, as already stated, 
has drawn attention to the remarkable branchial sense-organs of 
Limulus ; and Patten has pointed out that special organs, which he 
considers to be gustatory in function, are present on the mandibles 
of the prosomatic appsndages. I myself, as mentioned in my address 
to the British Association at Liverpool in 189G, searched for some 
special sense organ at the junction of the prosoma and mesosoma, 
and was rewarded by finding that that extraordinary adjunct to the 



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THE EVIDENCE OF THE AUDITORY APPARATUS 359 

last locomotor appendage, known as the flabellum, was an elaborate 
sense-organ. I now propose to show that all these special sense- 
organs are constructed on a somewhat similar plan ; that the structure 
of the branchial sense-organs suggests that they are organs for the 
estimation of water pressures ; that among air-breathing arthropods 
sense-organs, built up on a somewhat similar plan, are universally 
found, and are considered to be of the nature of auditory and equi- 
libration organs ; and, what is especially of importance, in view of 
the fact that the most prominent members of the Palaeostraca were 
the sea-scorpions, that the remarkable sense-organs of the scorpions 
known as the pectens belong apparently to the same group. 

The Poriferous Sense-Organs of the Appendages in Limulus. 

On all the branchial appendages in Limulus, special sense-organs 
are found of a most conspicuous character. They form in the living 
animal bluish convex circular patches, the situation of which on the 
appendages is shown in Fig. 58. These organs are not found on the 
non-branchial operculum. Gegenbaur, who was the first to describe 
them, has pointed out how the surface of the organ is closely set 
with chitinous goblets shaped as seen in Fig. 144, A, which do not 
necessarily project free on the surface, but are extruded on the 
slightest pressure. Each goblet fits into a socket in the chitinous 
covering, and is apparently easily protruded by variations of pressure 
from within. The whole surface of the organ on the appendage is 
slightly bulged in the living condition, and the chitin is markedly 
softer here than in the surrounding part of the limb. Each of these 
organs is surrounded by a thick protection of strongly branching 
spines. On the surface of the organ itself no spines are found, only 
these goblets, so that the surface-view presents an appearance as in 
Fig. 144, B. Each goblet possesses a central pore, which is the 
termination of a very fine, very tortuous, very brittle chitinous 
tubule (ch.t.), which passes from the goblet through the layers of the 
chitin into the subjacent tissue. The goblets vary considerably in 
size, a few very large ones being scattered here and there. The fine 
chitinous tubule is especially conspicuous in connection with these 
largest goblets. In the smaller ones there is the same appearance of 
a pore and a commencing tube, but I have not been able to trace the 
tube through the chitinous layers, as in the case of the larger goblets. 



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360 



THE ORIGIN OF VERTEBRATES 



Gegenbaur, in his picture, draws a straight tubule passing from every 
goblet among the fine canaliculi of the chitin. He says they are 
difficult to see, except in the case of the larger goblets. The tubule 
from the larger goblets is most conspicuous, and is in my sections 
always tortuous, never straight, as represented by Gegenbaur. A 
special branch of the appendage-nerve passes to these organs, and 




Fig. 144.— A, A Goblet from 
one or the Branchial Sense- 
Organs of Limulus (ch.t., 
chitiuoiiB tubule) ; B, Surface 
View of a Portion of a Bran- 
chial Sense-Organ. 




Fig. 145.— The Endognaths of Limulus 
pushed out of the way on one side 
in order to 8how the position of 
the flabellum (fl.) projecting to- 
wards the crack between the pro- 
somatic and mesosomatic carapaces. 



upon the fine branches of this nerve groups of ganglion-cells are seen, 
very similar in appearance to the groups described by Patten on the 
terminal branches of the nerves which supply the mandibular organs. 
At present I can see no mechanism by which the goblets are extruded 
or returned into place. In the case of the Capitellida?, Eisig describes 
retractor muscles by means of which the lateral sense-organs are 



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THE EVIDENCE OF THE AUDITORY APPARATUS 36 1 

brought below the level of the surface, and he imagines that the pro- 
trusion is effected by hydraulic means, by the aid of the vascular 
system. In the branchial sense-organs of Limulus there are no 
retractor muscles, and it seems to me that both retraction and pro- 
trusion must be brought about by alterations of pressure in the 
vascular fluids. Certainly the cavity of the organ is very vascular. 
If this be so, it seems likely enough that such an organ should be a 
very delicate organ for estimating changes in the pressure of the 
external medium, for the position of the goblets would depend on 
the relation between the pressure of the fluid inside the organ and 
that on the surface of the appendage. Whether the chitinous tubule 
contains a nerve-terminal or not I am unable to decide from. my 
specimens, but, judging from Patten's description of the similar 
chitinous tubules belonging to the mandibular organs, it is most 
highly probable that these tubules also contain a fine terminal 
nerve-fibre. 

These organs, then, represent segmental branchial sense-organs, 
of which it can be said their structure suggests that they may be 
pressure-organs ; but the experimental evidence is at present wanting. 

Passing now from the branchial to the prosomatic region, the 
first thing that struck me was the presence of that most conspicuous 
projection at the base of the last locomotor appendage, which is 
usually called the flabellum, and has been described by Lankester 
as an exopodite of this appendage. It is jointed on to the most basal 
portion of the limb (cf. Fig. 155), and projects dorsally from the limb 
into the open slit between the prosomatic and mesosomatic carapace, 
as is seen in Fig. 145 (/.). Of its two surfaces, the undermost is very 
convex and the uppermost nearly flat from side to side, the whole 
organ being bent, so that when the animal is lying half buried in 
sand, entirely covered over by the prosomatic and mesosomatic 
carapaces except along this slit between the two, the upper flat or 
slightly convex surface of the flabellum is exposed to any movement 
of water through this slit, and owing to its possessing a joint, the 
direction of the whole organ can be altered to a limited extent. The 
whole of this flat upper surface is one large sense-organ of a striking 
character, thus forming a great contrast to the convex under surface, 
which is remarkably free from tactile spines or special sense-organs. 

The nerve going to the flabellum is very large, almost as large 
as the nerve to the rest of the appendage, and the very large majority 



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362 



THE ORIGIN OF VERTEBRATES 



of the nerve-fibres turn towards the flat, uppermost side, where the 
sense-organ is situated. Between the nerve-fibres (n.) and the chi- 
tinous surface containing the special sense tubes masses of cells (gl.) 
are seen, as in Fig. 146, apparently nerve cells, which form a broad 
border between the nerve- fibres and the pigmented chitinogenous 




,-* %> 



^-•r** 







Fig. 147. — Section parallel to 
the Surface of Flabellum, 
showing the pobous termi- 
NATIONS of the Sense-Organs 
and the Arrangement of the 
Canaliculi round them. 



c\ \) J)J v n g* p ch 

Fig. 146.— Section through Flabellum. 

ch. } chitinous layers; s.o. } sense-organs; up., 
spike-organ ; p., pigment layer ; gl. y ganglion 
cell layer ; bl. and n. t blood-spaces and nerves. 

layer (p.). On the opposite side, nothing of the sort intervenes 
between the pigmented layer and the blood- spaces and nerve-fibres 
which constitute the central mass of the flabellum. 

At present I am inclined to look upon this mass of cells as 
constituting a large ganglion, which extends over the whole length 
and breadth of the upper surface of the flabellum. At the same 



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THE EVIDENCE OF THE AUDITORY APPARATUS 363 



cap 



cK 



< 



cKt 



V 



time, my preparations are not sufficiently clear to enable me to 
trace out the connections of these cells, especially their connections 
with the special sense- 
organs, r J 

In Fig. 148 I give a 
magnified representation 
of a section through three 
of these flabellar sense- 
organs. As is seen, the 
section divides itself into 
four zones: (1) the chi- 
tinous layer (ch.)\ (2) 
the layer of pigment (p.) 
and hypodermal cells ; 
(3) the layer of ganglion- 
cells (gl.)\ and (4) the 
layer of nerve -fibres (n.) 
and blood-spaces (&/.). 
The chitinous layer is 
composed of the usual 
three zones of the Li- 
mulus surface — exter- 
nally (Fig. 148), a thin 
homogeneous layer, fol- 
lowed by a thick layer 
of chitin (3), in which 
the fine vertical tubules 
or canaliculi are well 
marked ; the external 
portion (2) of this layer 
is differentiated from the 
rest by the presence of 
well - marked horizontal 
layers in addition to the 
canaliculi. 

In this chitinous layer 
the special sense-organs 
are found. They consist of a large tube which passes through all the 
layers of the chitin except the thin homogeneous most external layer. 




Fig. 148. 



6/., 



Section through three Sense- 
Organs of Flabellum. 

blood-spaces; n., nerve; gl., layer of ganglion- 
cells; p., pigment layer; ch., 1, 2, 3, the three 
layers of chitin ; ch.t., chitinous tubule in large 
tube of sense-organ ; cap., capitellum or swollen 
extremity of largo tube; can., very fine porous 
canals or canaliculi of chitin. 



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364 THE ORIGIN OF VERTEBRATES 

This tube is conical in shape, its base, which rests on the pigmented 
layer, being so large and the organs so crowded together that a section 
of the chitin across the base of the tubes gives the appearance of a 
honeycomb, the septa of which is all that remains of the chitin. 
This large tube narrows down to a thin elongated neck as it passes 
through the chitin, and then, at its termination, bulges out again 
into an oval swelling (cap.) situated always beneath the homogeneous 
most external layer of chitin. Within this tube a fine chitinous 
tubule (ch. t.) is situated similar to that seen in the branchial sense- 
organs ; it lies apparently free in the tube, not straight, but sinuous, 
and it passes right through all the chitinous layers to open at the 
surface as a pore; in the last part of its course, where it passes 
through the most external layer (1) of chitin, it lies always at right 
angles to the surface. 

If the flabellum be stained with methylene blue and acid fuchsin, 
then all the canaliculi in the chitin show up as fine red lines, and 
present the appearance given in Fig. 148, and it is seen that each 
of the terminations of the tubules is surrounded in the homogeneous 
layer of chitin by a thick-set circular patch of canaliculi which pass 
to the very surface of the chitin, while the canaliculi in other parts 
terminate at the commencement of the homogeneous layer and do 
not reach the surface. Further, the contents of the oval swelling, 
and, indeed, of the tube as a whole, are stained blue, the chitinous 
tubule being either unstained or slightly pink in colour. We see, 
then, that the chitinous tubule alone reaches the surface, while the 
large tube, which contains the tubule, terminates in an oval swelling, 
which often presents a folded or wrinkled appearance, as in Fig. 149 
(see also Patten's Fig. 1, Plate I.). This terminal bulging of the 
tube is reminiscent of the bulging in the chitinous tubes of the lyri- 
form organs of the Arachnida, as described by Gaubert, and of the 
poriferous chordotonal organs in insects, as described by Graber (see 
Fig. 150). This terminal swelling is filled with a homogeneous 
refringent mass staining blue with methylene blue, in which I have 
seen no trace of a nucleus ; through this the chitinous tubule makes 
its way without any sign of bulging on its part. Patten, in his 
description of the sense-organs on the mandibles of Limulus, which 
are evidently the same in structure as those on the flabellum, refers 
to this homogeneous mass as a coagulum. I doubt whether this 
is an adequate description ; it appears to me to stain rather more 



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THE EVIDENCE OF THE AUDITORY APPARATUS 365 

readily than a blood-coagulum, yet in the sense of being structure- 
less it resembles a coagulum. 

The enormous number of these organs crowded together over 
the whole flat surface of the flabellum produces a very striking 
appearance when viewed on the surface. Such a view presents an 
appearance resembling that of the surface-view of the branchial sense- 
organs; in both cases the surface is covered with a great number 
of closely set circular plaques, in the centre of each of which is seen 
a well-marked pore. The circular plaques in the case of the flabellum 
are much smaller than those of the branchial sense-organs, and 
clearly are not protrusible as in the latter organs, the appearance as 
of a plaque being due to the ring of thickly-set canaliculi round the 
central tubule, as already described. When stained with methylene 
blue, the surface view of the flabellum under a low power presents 
an appearance of innumerable circular blue masses, from each of 
which springs a fine bent hair, terminating in a pore at the surface. 
The blue masses are the homogeneous substance (cap.) of the bulgings 
seen through the transparent external layer of chitin, and the hairs 
are the terminal part of the chitinous tubules. Patten has repre- 
sented their appearance in the mandibles in his Fig. 2, Plate I. 

The large tubes in the chitin alter in shape according to their 
position. Those in the middle of the sensory surface of the flabellum, 
in their course through the chitinous layers, are hardly bent at all ; 
as they approach the two lateral edges of this surface, their long thin 
neck becomes bent more and more, the bending always being directed 
towards the middle of the surface (see Fig. 146) ; in this way the 
chitinous tubules increase more or less regularly in length from 
the centre of the organ to the periphery. The large basal part of 
the conical tube contains, besides the chitinous tubule, a number 
of nuclei which are confined to this part of the tube ; some of these 
nuclei look like those belonging to nerve-fibres, others are apparently 
the nuclei of the chitinogenous membrane lining the tube. I have 
never seen any sign of nerve-cells in the tube itself. 

The only other kind of sense-organ I have found in connection 
with these sense-organs are a few spike-like projections, the appear- 
ance of which is given in Fig. 149. I have always seen these in the 
position given in Fig. 146 (sp.), i.e. at the junction of the surface 
which contains the sense-organs and the surface which is free from 
them. They are, so far as I have seen, not very numerous ; I have 



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CA.e 



366 THE ORIGIN OF VERTEBRATES 

not, however, attempted to examine the whole sense-organ for the 
purpose of estimating their number and arrangement. 

As is seen in Fig. 149, they possess a fine tubule of the same 
character as that of the neighbouring sense-organs, which apparently 
terminates at the apex of the projecting spike. They appear to 
belong to the same group as the other poriferous sense-organs, and 
are of special interest, because in their appearance they form a link 
between the latter and the poriferous sense-organs which charac- 
terize the pecten of the scorpion (c/. Fig. 152, C). 

Such, then, is the structure of this remarkable sense-organ of the 
flabellum, as far as I have been able to work it out with the materials 

at my disposal. It is 
evident that the flabellar 
organs, apart from the 
spike-organs, are of the 
same kind as those de- 
scribed by Patten on the 
mandibles and chelse of 
Limulus, and therefore it 
is most probable that the 
nerve - terminals in the 
chitinous tubules, and 
the origin of the latter, 
are similar in the two sets 
of organs. 

These organs, as Patten 
has described them, are 
situated in lines on the 
spines of the mandibles of the prosomatic locomotor appendages, 
and are grouped closely together to form a compact sense orgau 
on the surface of the inner mandible (Lankester s epicoxite) (i.m. 
in Fig. 155), so that a surface-view of the organ here gives 
the characteristic appearance of these poriferous sense-patches. 
Precisely similar organs are found on the chilaria, which are, in 
function at all events, simply isolated mandibles, to use Patten's 
terminology. 

On the digging appendage (ectognath), as the comparison of 
Fig. 155, A and C, shows, the mandibular spines are almost non- 
existent, and the inner mandible or epicoxite is not present, so that 




Fig. 149.— Spike-Organ of Flabellum. 
ch.t.. chitinous tubule. 



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THE EVIDENCE OF THE AUDITORY APPARATUS 367 

the special sense-organ of this appendage is represented solely by the 
flabellum. 

This sketch of the special sense-organs of Limulus shows that all 
the appendages of Limulus possess special sense-organs, with the 
exception of the operculum. All these sense-organs are formed on 
the same plan, in that they possess a fine chitinous tubule passing 
through the layers of chitin into the underlying hypodermal and 
nervous tissues, which terminates on the surface in a pore. The sur- 
face of the chitin where these pores are situated is perfectly smooth, 
although, in the case of the branchial sense-organs, the goblet-shaped 
masses of chitin, each of which contains a pore, are able to be pressed 
out beyond the level of the surface. 

As to their functions, we unfortunately do not know much that 
is definite. Patten considers that he has evidence of a gustatory 
function in the case of the mandibular organs, and suggests also a 
temperature-sense in the case of some of these organs. The large 
organ of the flabellum and the branchial organs he has not taken into 
consideration. The situation of these organs puts the suggestion of 
any gustatory function, as far as they are concerned, out of the ques- 
tion ; and I do not think it probable that such large specialized organs 
would exist only for the estimation of temperature, when one sees 
how, in the higher animals, the temperature-nerves and the nerves of 
common sensation are universally distributed over the body. As 
already stated, the structure of the branchial organs seems to me to 
point to organs for estimating varying pressures more than anything 
else, and I am strongly inclined to look upon the whole set of organs 
as the derivatives of the lateral sense-organs of annelids, such as are 
described by Eisig in the Capitellidae. This is Patten's opinion with 
respect to the mandibular organs ; and from what I have shown, 
these organs cannot be separated in type of structure from those of 
the flabellum and the branchial sense-organs. 

In our search, then, for the origin of the vertebrate auditory organ 
in Limulus and its allies, we see so far the following indications : — 

1. The auditory organ of the vertebrate is regarded as a special 
organ belonging to a segmentally arranged set of lateral sense-organs, 
whose original function was co-ordination and equilibration. 

2. Such a set of segmentally arranged lateral sense-organs is 
found in annelids in connection with the dorsal cirri of the ventral 
parapodia. 



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368 THE ORIGIN OF VERTEBRATES 

3. If, as has been supposed, there is a genetic connection between 
(1) and (2) and if, as I suppose, the vertebrates did not arise from 
the annelids directly, but from a protostracan group, then it follows 
that the lateral sense-organs, one of which gave rise to the auditory- 
organ, must have been situated on the protostracan appendages. 

4. In Limulus, which is the sole surviving representative of the 
palseostracan group, such special sense-organs are found on both the 
prosomatic and mesosomatic appendages, and therefore may be 
expected to give a direct clue to the origin of the vertebrate auditory 
organ. 

5. Both from its position, its size, and its specialization, the 
flabellum, i.e. an organ corresponding to the flabellum, must be 
looked upon as more likely to give a direct clue to the origin of the 
auditory organ than the sense-organs of the branchial appendages, or 
the so-called gustatory organs of the mandibles. 

The Auditory Organs of Arachnids and Insects. 

The difficulty of the investigating these organs consists in the fact 
that so little is known about them in those Arthropoda which live in 
the water ; the only instance of any organ apparently of the nature 
of an auditory organ, is the pair of so-called auditory sacs at the base 
of the antenna? in various decapods. We are in a slightly better 
position when we turn to the land-living arthropods ; here the pre- 
sence of stridulating organs in so many instances carries with it the 
necessity of an organ for appreciating sound. It has now been shown 
that such stridulating organs are not confined to the Insecta, but are 
present also in the scorpion group, and I myself have added to their 
number by the discovery of a distinct stridulating apparatus in 
various members of the Phrynidae. We may then take it for granted 
that arachnids as well as insects hear. Where is the auditory organ ? 

Many observers believe that certain surface-organs found uni- 
versally among the spiders, to which Gaubert has given the name of 
lyriform organs, are auditory in function. His investigations show 
that they are universally present on the limbs and pro-meso-sternite 
of all spiders ; that they are present singly, not in groups, on the 
limbs of Thelyphonus, and that a group of them exists on the second 
segment of each limb in the members of the Phrynus tribe. In the 
latter case this organ is the most elaborate of all described by him. 



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THE EVIDENCE OF THE AUDITORY APPARATUS 369 

It is especially noticeable that they do not exist in Galeodes or 
in the scorpions, but in the former special sense-organs are found in 
the shape of the so-called * racquet-organs/ on the basal segments of 
the most posterior pair of appendages, and also, according to Gaubert, 
on the extremity of the palps and the first pair of feet, while in the 
latter they occur in the shape of the pectens. 

This observation of Gaubert suggests that the place of the 
lyriform organs in other arachnids is taken in Galeodes by the 
racquet-organs, and in the scorpions by the pectens. Bertkau, 
Schimk^witsch, and Wagner, as quoted by Gaubert, all suggest that 
the lyriform organs of the arachnids belong to the same group of 
sense-organs as the porous chordotonal organs of the Insecta, sense- 
organs which have been found in every group of Insecta, and are 
generally regarded as auditory organs. Gaubert does not agree with 
this, and considers the lyriform organs to be concerned with the 
temperature-sense rather than with audition. 

The chordotonal organs of insects have been specially studied by 
Graber. He divides them into two groups, the poriferous and the 
non-poriferous, the former being characterized by the presence of 
pores on the surface arranged in groups or lines. These poriferous 
chordotonal organs are remarkably constant in position, being found 
only at the base of the wings on the subcostal ridge, in marked 
contrast to the other group of chordotonal organs which are found 
chiefly on the appendages in various regions. The striking character 
of this fixity of position of these organs and the universality of their 
presence in the whole group, led Graber to the conclusion that in 
these poriferous chordotonal organs we are studying a form of 
auditory apparatus which characterized the ancestor of the insect- 
group. These organs are always well developed on the hind wings, 
and in the large group of Diptera the auditory apparatus has usurped 
the whole of the function of the wing ; for the balancers or ' halteres,' 
as they are called, are the sole representatives of the hind wings, and 
they are usually considered to be of the nature of auditory organs. 
It is instructive to find that such an auditory organ serves not only 
for the purpose of audition, but also as an organ of equilibration ; 
thus Lowne gives the evidence of various observers, and confirms it 
himself, that removal of the balancers destroys the power of orderly 
flight in the animal. 

A striking peculiarity of these organs in the Insecta, as described 

2 B 



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37o 



THE ORIGIN OF VERTEBRATES 



by Graber, is the bulging of the porous caual near its termination 
(Fig. 150, C). This bulging is filled with a homogeneous, highly 
refractive material, from which, according to Lowne, a chordotonal 
thread passes, to be connected with a ganglion-cell and nerve. 
This sphere of refractive material he calls the 'capo^eUum' of the 
chordotonal thread. The presence of this material produces in a 
surface view an appearance as of a halo around the terminal plaque 
with its central pore ; Graber has attempted to represent this by the 
white area round the central area (in Fig. 150, B). A very similar 
appearance is presented by the surface view of the flabellum in 
those parts where the tube runs straight to the surface, so that the 



5.0 





B 

Fig. 150 (from Gbabeb). — A, Section op Subcostal Nebvube of Hind Wing of 
Dytiscus to show patch of Pobifebous Obgans (s.o.). B, Subface View of 
Pobifebous Obgans; the White Space bound each Obgan indicates the 

DEEPER LYING ReFBINGENT BODY WHICH FILLS THE BULGING OF THE CANAL 

seen in Tbansvebse Section in C. 



refractive material which fills the oval bulging shines through the 
overlying chitin and appears to surround the terminal plaque with a 
translucent halo. 

Such a peculiarity must have a very definite meaning, and sug- 
gests that the canals in the flabellum of Limulus and in the hind 
wings of insects belong to the same class of organ, the chitinous 
tubule with its nerve-terminal in the former corresponding to the 
chordotonal thread in the latter. One wonders whether this sphere 
of refractive material or ' capitellum ' (to use Lowne's phraseology) 
is so universally present in order to act as a damper upon the 
vibrations of the chordotonal thread in the one case and of the 



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THE EVIDENCE OF THE AUDITORY APPARATUS 37 1 

chitinous tubule in the other, just as the nwmbrana tectoria and the 
otoliths act in the case of the vertebrate ear. 

Patten says that the only organs which seem to him to be compar- 
able with the gustatory porous organs of Limulus are the sense-organs 
in the extremities of the palps and of the first pair of legs of Galeodes, 
as described by Gaubert. I imagine that he was thinking only of 
arachnids, for the comparison of his drawings with those of Graber 
show what a strong family resemblance exists between the poriferous 
sense-organs of Limulus and those of the insects. On the course 
of the terminal nerve-fibres, between the nerve-cell and their entrance 
into the porous chitinous canal, Graber describes the existence of 
rods or scolophores. On the course of the terminal fibres in the 
Limulus organ, between the nerve-cells and their entrance into the 
porous chitinous canal, Patten describes a spindle-shaped swelling, 
containing a number of rod-like thickenings among the fibrils in the 
spindle, which present an appearance reminiscent of the rods described 
by Graber. 

It appears as though a type of sense-organ, characterized by the 
presence of pores on the surface and a fine chitinous canal which 
opens at these pores, was largely distributed among the Arthropoda. 
According to Graber, this kind of organ represents a primitive type 
of sense-organ, which was probably concerned with audition and 
equilibration, and he expresses surprise that similar organs have not 
been discovered among the Crustacea. It is, therefore, a matter of 
great interest to find that so ancient a type of animal as Limulus, 
closely allied to the primitive crustacean stock, does possess pori- 
ferous sense-organs upon its appendages which are directly compar- 
able with these poriferous chordotonal organs of the Insecta. 

The Pectens of Scorpions. 

Among special sense-organs such as those with which I am now 
dealing, the pectens of scorpions and the * racquet-organs ' of Gale- 
odes must, in all probability, be classed. I have given my reasons 
for this conclusion in my former paper. 1 At present such reasons 
are based entirely upon the structure of the organs; experimental 

1 " The Origin of Vertebrates, deduced from the Study of Ammoccetes." Part X., 
" The Origin of the Auditory Organ : the Meaning of the VHIth Cranial Nerve." 
Journ. AnaU and Physiol, vol. 86, 1902. 



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372 



THE ORIGIN OF VERTEBRATES 



evidence as to their function is entirely wanting. With respect to the 
pectens of the scorpion (Fig. 151), it has been suggested that they 
are of the nature of copulatory organs, a suggestion which may be 
dismissed without hesitation, for they are not constructed after the 
fashion of claspers, but are simply elaborate sense-organs, and, as 

such, are found equally in male or female. 
The only observer who has hitherto 
specially studied the structure of the 
sense-organs in the pecten is, as far as 
I know, Gaubert, and he describes their 
structure together with that of the sense- 
organs of the racquets of Galeodes, in 
connection with the lyriform organs of 
arachnids, as though he recognized a 
family resemblance between the three 
sets of organs. 

The pecten of the scorpions is an 
elaborate sense-organ, or rather group of 
sense-organs, the special organ being 
developed on each tooth of the comb; 
its surface, which is frequently flattened, 
being directed backwards and inwards, 
when the axis of the pecten is horizontal 
at right angles to the length of the body. 
The surface view of this part of the tooth 
resembles that of the branchial organs or 
of the flabellum in Limulus, in that it 
is thickly covered with circular patehes, 
in the centre of which an ill-defined 
appearance as of a fine pore is seen. In Fig. 152, B, I give a sketch 
of the surface view of a part of the organ. 

Transverse sections of a tooth of the comb of Scorpio Europceus 
present the appearance given in Fig. 152, A, and show that each 
of these circular patches is the surface-view of a goblet-shaped 
chitinous organ, Fig. 152, C, from the centre of which a short, some- 
what cylindrical chitinous spike projects. Within this spike, and 
running through the goblet into the subjacent tissue, is a fine 
tubule. The series of goblets gives rise to the appearance of the 
circular plaques on the surface-view, while the spike with its tubule 




Fig. 151. — Undeb Surface of 
Scorpion (Androctonus). 

The operculum is marked out 
with dots, and on each side 
of it is seen one of the pec- 
tens. 



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THE EVIDENCE OF THE AUDITORY APPARATUS 373 

is the cause of the ill-defined appearance of the central pore, just 
as the terminal pore is much less conspicuous on surface-view in the 
spike-organs of the flabellum than in the purely poriferous organs, 
no part of which projects beyond the level of the chitinous surface. 

The fine tubule is soon lost in the thickened but soft modification 
of the chitinous layer (ch.) which is characteristic of the sense-organ ; 
at all events, I have not succeeded in tracing it through this layer 
with any more success than in the corresponding case of the tubules 




Fig. 152.— A, Section throigh Tooth of Pecten op Scorpion; B, Surface View 
of Sense-Organs; C, Goblet of Sense-Organ more highly magnified. 

bl. and n., region of blood-spaces and nerves ; gl., ganglion-cell layer ; ch. t modified 
chitinous layer; 3.0., sense-organ. 

belonging to the smaller goblets of the branchial sense-organ of 
Limulus already described. 

At the base of the modified chitinous layer a series of cells is 
seen, many, if not all, of which belong to the chitinogenous layer. 
Next to these is the marked layer of ganglion-cells (gl.), similar to 
those seen in the flabellum of Limulus. The rest of the space in the 
section of the tooth is filled up with nerves (n.) and blood-spaces (bl) 
just as in the section, Fig. 146, of the flabellum of Limulus. 

Gaubert does not appear to have seen the goblets at all clearly ; 



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374 



THE ORIGIN OF VERTEBRATES 



he describes them simply as conical eminences, and states that they 
" recouvrent un pore analogue a celui des poils mais plus petit ; 
il est rempli par le protoplasma de la couche hypodermique." 
From the ganglion, according to him, nervous prolongations pass, 
which traverse the chitinogenous layer and terminate at the base 
of the conical eminences. Each of these prolongations " presente 
sur son trajet, mais un peu plus pres du ganglion que de sa termi- 
naison peripherique, une cellule nerveuse fusiforme (g.) offrant, 
comme celles du ganglion, un gros noyau." He illustrates his 
description with the following, Fig. If) 3, 
taken from his paper. 

I have not been able to obtain any evi- 
dence of a fusiform nerve-cell on the course 
of the terminal nerve-fibres as depicted by 
him ; fusiform cells there are in plenty, as 
depicted in my drawing, but none with a 
large nucleus resembling those of the main 
ganglion. In no case, either in the flabellum 
or in the branchial organs of Limulus, or in 
the pecten-organs, have I ever seen a ganglion- 
cell within the chitin-layer ; all the nuclei 
seen there resemble those of the' cells of 
the hypodermis or else the elongated nuclei 
characteristic of the presence of nerve-fibres. 
Gaubert's drawing is a striking one, and I 
have looked through my specimens to see 
whether there was anything similar, but have 
hitherto failed to obtain any definite evidence 
of anything of the kind. 
I feel, myself, that an exhaustive examination of the structure 
and function of the pecten of scorpions ought to be undertaken. At 
present I can only draw the attention of my readers to the similarity 
of the arrangement of parts, and of the nature of the end-organs, in 
the sense-organs of the flabellum of Limulus and of the pecten of 
the scorpion. In both cases the special nerve-fibres terminate in 
a massive ganglion, situated just below the chitinogenous layer. In 
both cases the terminal fibres from these ganglion-cells pass through 
the modified chitinous layer to supply end-organs of a striking cha- 
racter ; and although the end-organ of the pecten of the scorpion does 




Fig. 153 (from Gaubert). 
— Section op a Tooth 
of Pecten of Scorpion. 

n., none; gl., ganglion. 



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THE EVIDENCE OF THE AUDITORY APPARATUS 375 

not closely resemble the majority of the end-organs of the flabellum, 
yet it does resemble, on the one hand, the isolated poriferous spikes 
found on the flabellum (Fig. 149) and, on the other, the poriferous 
goblets found on the sense-patches of the branchial appendages of 
Limulus (Fig. 144, A), so that a combination of these two end-organs 
would give an appearance very closely resembling that of the pecten 
of the scorpion. 

Finally, the special so-called ' racquet-organs ' of Galeodes, which 
are found on the most basal segments of the last pair of prosomatic 
appendages, ought also to be considered here. Gaubert has described 
their structure, and shown how the nerve-trunk in the handle of the 
racquet splits up into a great number of separate bundles, which 
spread out fan-shaped to the free edge of the racquet ; each of these 
separate bundles supplies a special sense-organ, which terminates 
as a conical eminence on the floor of a deep groove, running round 
the whole free edge of the racquet. This groove is almost converted 
into a canal, owing to the projection of its two sides. Gaubert 
imagines that the sense-organs are pushed forward out of the groove 
to the exterior by the turgescence of the whole organ ; each of the 
nerve-fibres forming a bundle is, according to Gaubert, connected 
with a nerve-cell before it reaches its termination. 

This sketch of the special sense-organs on the appendages of 
Limulus, of the scorpions, of Galeodes, and other arachnids, and their 
comparison with the porous chordotonal organs of insects, affords 
reason for the belief that we are dealing here with a common group 
of organs, which, although their nature is not definitely known, 
have largely been accredited with the functions of equilibration and 
audition, a group of organs among which the origin of the auditory 
organ of vertebrates must be sought for, upon any theory of the 
origin of vertebrates from arthropods. 

Whenever in any animal these organs are concentrated together 
to form a special organ, it is invariably found that the nerve going^to 
this organ is very large, out of all proportion to the size of the organ, 
and also that the nerve possesses, close to its termination in the 
organ, large masses of nerve-cells. Thus, although the whole hind 
wing in the blow-fly has been reduced to the insignificant balancers 
or ' halteres/ yet, as Lowne states, the nerves to them are the largest 
in the body. 

The pectinal nerve in the scorpion is remarkable for its size, and 



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376 THE ORIGIN OF VERTEBRATES 

so, also, is the nerve to the flabellum in Limulus, while the large size 
of the auditory nerve in the vertebrate, in distinction to the size of 
the auditory apparatus, has always aroused the attention of anatomists. 

Throughout this book my attention has been especially directed 
to both Limulus and the scorpion group in endeavouring to picture to 
myself the ancestor of the earliest vertebrates, because the Eury- 
pteridae possessed such marked scorpion-like characteristics ; so that in 
considering the origin of a special sense-organ, such as the vertebrate 
auditory organ near the junction of the prosoma and mesosoma, it 
seems to me that the presence of such marked special sense-organs as 
the flabellum on the one hand and the pecten on the other, must 
both be taken into account, even although the former is an adjunct 
to a prosomatic appendage, while the latter represents, according to 
present ideas, the whole of a mesosomatic appendage. 

From the point of view that the VIITth nerve represents a 
segment immediately posterior to that of the Vllth, it is evident 
that an organ in the situation of the pecten, immediately posterior 
to the operculum, i.e. according to my view, posterior to the segment 
originally represented by the Vllth nerve, is more correctly situated 
than an organ like the flabellum, which belongs to a segment anterior 
to the operculum. 

On the other hand, from the point of view of the relationship 
between the scorpions and the king-crabs, it is a possibly debatable 
question whether the pecten really belongs to a segment posterior to 
the operculum. The position of any nerve in a series depends upon its 
position of origin in the central nervous system, rather than upon the 
position of its peripheral organ. Now, Patten gives two figures of the 
brain of the scorpion built up from serial sections. In both he shows 
that the main portion of the pectinal nerve arises from a swelling, to 
which he gives the name ganglion nodosum. This swelling arises on 
each side in close connection with the origin of the most posterior 
prosomatic appendage-nerve, according to his drawings, and posteriorly 
to such origin he figures a small nerve which he says supplies the 
distal parts of the sexual organs. This nerve is the only nerve which 
can be called the opercular nerve, and apparently arises posteriorly 
to the main part of the pectinal nerve. If this is so, it would 
indicate that the pectens arose from sense-organs which were origi- 
nally, like the flabella, pre-opercular in position, but have shifted to 
a post-opercular position. 



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THE EVIDENCE OF THE AUDITORY APPARATUS 377 



The Origin of the Parachordals and Auditory Cartilaginous 

Capsule. 

In addition to what I have already said, there is another reason 
why a special sense-organ such as the pecten is suggestive of the 
origin of the vertebrate auditory organ, in that such a suggestion 
gives a clue to the possible origin of the parachordals and auditory 
cartilaginous capsules. 

In the lower vertebrates the auditory organ is characterized by 
being surrounded with a cartilaginous capsule which springs from 
a special part of the axial cartilaginous skeleton on each side, known 
as the pair of parachordals. The latter, in Ammocoetes, form a 
pair of cartilaginous bars, which unite the trabecular bars with the 
branchial cartilaginous basket-work. They are recognized throughout 
the Vertebrata as distinct from the trabecular bars, thus forming 
a separate paired cartilaginous element between the trabecule© and 
the branchial cartilaginous system, which of itself indicates a position 
for the auditory capsule between the prosomatic trabeculae and the 
mesosomatic branchial cartilaginous system. 

The auditory capsule and parachordals when formed are made of 
the same kind of cartilage as the trabecule, i.e. of hard cartilage, and 
are therefore formed from a gelatin-containing tissue, and not from 
muco-cartilage. Judging from the origin already ascribed to the 
trabecule, viz. their formation from the great prosomatic entochon- 
drite or plastron, this would indicate that a second entochondrite 
existed in the ancestor of the vertebrate in the region of the junction 
of the prosoma and mesosoraa, which was especially connected with 
the sense-organ to which the auditory organ owes its origin. This 
pair of entochondrites becoming cartilaginous would give origin to 
the parachordals, and subsequently to the auditory capsules, their 
position being such that the nerve to the operculum would be 
surrounded at its origin by the growth of cartilage. 

On this line of argument it is very significant to find that 
the scorpions do possess a second pair of entochondrites, viz. the 
supra-pectinal entochondrites, situated between the nerve- cord and 
the pectens, so that if the ancestor of the Cephalaspid was sufficiently 
scorpion-like to have possessed a second pair of entochondrites and 
at the same time a pair of special sense-organs of the nature either of 



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378 THE ORIGIN OF VERTEBRATES 

the pectens or flabella, then the origin of the auditory apparatus 
would present no difficulty. 

It is also easy to see that the formation of the parachordals from 
entochondrites homologous with the supra-pectinal entochondrites, 
would give a reason why the Vllth or opercular nerve is involved 
with the VXIIth in the formation of the auditory capsule, especially 
if the special sense-organ which gave origin to the auditory organ 
was originally a pre-opercular sense-organ such as the flabellum, 
which subsequently took up a post-opercular position like that of 
the pecten. 

The Evidence of Ammoccrtes. 

As to the auditory apparatus itself, we see that the elaborate 
organ for hearing — the cochlea — has been evolved in the vertebrate 
phylum itself. In the lowest vertebrates the auditory apparatus 
tends more and more to resolve itself into a simple epithelial sac, the 
walls of which in places bear auditory hairs projecting into the sac, 
and in part form otoliths. Such a simple sac forms the early stage 
of the auditory vesicle in Ammoccetes, according to Shipley ; subse- 
quently, by a series of foldings and growings together, the chambers of 
the ear of the adult Petromyzon, as figured and described by Ketzius, 
are formed. Further, we see that throughout the Vertebrata this sac 
was originally open to the exterior, the auditory vesicle being first 
an open pit, which forms a vesicle by the approximating of its sides, 
the last part to close being known as the recessus labyrinthicm ; in 
many cases, as in elasmobranchs, this part remains open, or com- 
municates with the exterior by means of the ductus endolymphaticus. 

Judging, therefore, from the embryological evidence, it would 
appear that the auditory organ originated as a special sense-organ, 
formed by modified epithelial cells of the surface, which epithelial 
surface becoming invaginated, came to line a closed auditory vesicle 
under the surface. This special sense-organ was innervated from 
a large ganglionic mass of nerve-cells, situated close against the 
peripheral sense-cells, the axis-cylinder processes of which formed 
the sensory roots of the nerve. 

Yet another peculiarity of striking significance is seen in connec- 
tion with the auditory organ of Ammoccetes. The opening of the 
cartilaginous capsule towards the brain is a large one (Fig. 154), and 



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THE EVIDENCE OF THE AUDITORY APPARATUS 379 

admits the passage not only of the auditory and facial nerves, but 
also of a portion of the peculiar tissue which surrounds the brain. 
The large cells of this tissue, with their feebly staining nuclei and 
the pigment between them, make them quite unmistakable ; and, as 
I have already stated, nowhere else in the whole of Ammocoetes is 
such a tissue found. When I first noticed these cells within the 
auditory capsule, it seemed to me almost impossible that my inter- 
pretation of them as the remnant of the generative and hepatic tissue 
which surrounds the brain of animals such as Limulus could be true, 
for it seemed too unlikely that a part of the generative system could 




vin 



g«n Hi hi K er » 

Fig. 154. — Transverse Section through Auditory Capsut.es and Brain op 

Ammoccktks. 
Ah., auditory organ; VIIl y auditory nerve; gl., ganglion cells of VHIth nerve; 

An. cart. t cartilaginous auditory capsule; gcn. t cells of old generative tissue 

round brain and in auditory capsule ; bl., blood-vessels. 

ever have become included in the auditory capsule. Still, they are 
undoubtedly there ; and, as already argued with respect to the 
substance round the brain, they must represent some pre-existing 
tissue which was functional in the ancestor of Ammoco?tcs. If my 
interpretation is right, this tissue must be generative and hepatic 
tissue, and its presence in the auditory capsule immediately becomes 
a most important piece of evidence, for it proves that the auditory 
organ must have been originally so situated that a portion of the 
generative and hepatic mass surrounding the cephalic region of 
the nervous system followed the auditory nerve to the peripheral 
sense-organ. 



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380 THE ORIGIN OF VERTEBRATES 

Here there was a test of the truth of my theory ranking second 
only to the test of the median eyes ; the strongest possible evidence 
of the truth of any theory is given when by its aid new and unex- 
pected facts are brought to light. The theory said that in the group 
of animals from which the vertebrates arose, a special sense-organ 
of the nature of an auditory organ must have existed on the base of 
one of the appendages situated at the junction of the prosoma and 
mesosoma, and that into this basal part of the appendage a portion 
of the cephalic mass of generative and hepatic material must have 
made its way in close contiguity to the nerve of the special 
organ. 

The only living example which nearly approaches the ancient 
extinct forms from which, according to the theory, the vertebrates 
arose, is Limulus, and, as has already been shown, in this animal, in 
the very position postulated by the theory, a large special sense- 
organ — the flabellum— exists, which, as already stated, may well 
have given rise to a sense-organ concerned with equilibration and 
audition. If, further, it be found that a diverticulum of the gene- 
rative and hepatic material does accompany the nerve of the 
flabellum in the basal part of the appendage, then the evidence 
becomes very strong that the auditory organ of Ammoccetes, i.e. of 
the ancient Cephalaspids, was derived from an organ homologous 
with the flabellum ; that, therefore, the material round the brain of 
Ammoccetes was originally generative and hepatic material ; that, in 
fact, the whole theory is true, for all the parts of it hang together so 
closely that, if one portion is accepted, all the rest must follow. As 
pointed out in my address at Liverpool, and at the meeting of the 
Philosophical Society at Cambridge, it is a most striking fact that a 
mass of the generative and hepatic tissue does accompany the flabellar 
nerve into the basal part of this appendage. Into no other appendage 
of Limulus is there the slightest sign of any intrusion of the gene- 
rative and hepatic masses ; nowhere, except in the auditory capsule, 
is there any sign of the peculiar large-celled tissue which surrounds 
the brain and upper part of the spinal cord of Ammoccetes. The 
actual position of the flabellum on the basal part of the ectognath is 
shown in Fig. 155, A, and in Fig. 155, B, I have removed the chitin, 
to show the generative and hepatic tissue (gen.) lying beneath. 

The reason why, to all appearance, the generative and hepatic 
mass penetrates into the basal part of this appendage only is apparent 



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THE EVIDENCE OF THE AUDITORY APPARATUS 381 

wheu we see (as Patten and Redenbaugh have pointed out) to what 
part of the appendage the flabelluin in reality belongs. 

Patten and Eedeubaugh, in their description of the prosomatic 
appendages of Limulus, describe the segments of the limbs as (1) the 




Fig. 155.— A, The Digging Appendage ou Ectognath of Limulus; B, The 
Middle Protuberance (2) of the Entocoxite opened, to show the 
Generative and Hepatic Tissue (gen.) within it ; C, One of the Proso- 
matic Locomotor Appendages or Endognaths of Limulus, for comparison 
with A. 

Ji. t nabellum; cox., coxopodite; ent., entocoxite; w., mandible ; i.m., inner mandible 

or epicoxite. 

dactylopodite, (2) the propodite, (3) the inero- and carpo-podites, 
(4) the ischiopodite, (5) the basipodite, and (6) the coxopodite (cox. 
in Fig. 155). Still more basal than the coxopodite is situated the 
entocoxite (cnt. in Fig. 155), which is composed of three sclerites 



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382 THE ORIGIN OF VERTEBRATES 

or sensory knobs, to use Patten's description. The middle one of 
these three sclerites enlarges greatly in the digging appendage, and 
grows over the coxopodite to form the base from which the flabellum 
springs. Thus, as they have pointed out, the flabellum does not 
belong to the coxopodite of the appendage, but to the middle sensory 
knob of the entocoxite. Upon opening the prosomatic carapace, 
it is seen that the cephalic generative and hepatic masses press 
closely against the internal surface of the prosomatic carapace and 
also of the entocoxite, so that any enlargement of one of the sensory 
knobs of the entocoxite would necessarily be filled with a protrusion 
of the generative and hepatic masses. This is the reason why the 
generative and hepatic material apparently passes into the basal 
segment of the ectognath, and not into that of the endognaths ; it 
does not really pass into the coxopodite of the appendage, but into 
an enlarged portion of the entocoxite, which can hardly be considered 
as truly belonging to the appendage. Kishinouye has stated that 
a knob arises in the embryo at the base of each of the prosomatic 
locomotor appendages, but that this knob develops only in the last 
or digging appendage (ectognath) forming the flabellum. Doubtless 
the median sclerites of the entocoxites of the endognaths represent 
Kishinouye's undeveloped knobs. 

I conclude, therefore, that the flabellum, together with its basal 
part, is an adjunct to the appendage rather than a part of it, and 
might, therefore, easily remain as a separate and well-developed 
entity, even although the appendage itself dwindled down to a 
mere tentacle. 

The evidence appears to me very strong that the flabellum of 
Limulus and the pecten of scorpions are the most likely organs to 
give a clue to the origin of the auditory apparatus of vertebrates. 
At present both the Eurypterids and Cephalaspids have left us in the 
lurch ; in the former there is no sign of either flabellum or pecten ; 
in the latter, no sign of any auditory capsule beyond Rohon's dis- 
covery of two small apertures situated dorsally on each side of the 
middle line in Tremataspis, which he considers to be the termination 
of the ductus endolymphatic us on each side. In both cases it is 
probable, one might almost say certain, that any such special 
sense-organ, if present, was not situated externally, but was sunk 
below the surface as in Ammocoetes. 

The method by which such a sense-organ, situated externally on 



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THE EVIDENCE OF THE AUDITORY APPARATUS 383 

the surface of the animal, comes phylogenetically to form the lining 
wall of an internally situated membranous capsule is given by the 
ontogeny of this capsule, which shows step by step how the sense- 
organ sinks in and forms a capsule, and finally is entirely removed 
from the surface except as regards the ductus cndolympkaticus. 



Summary. 

The special apparatus for hearing- is of a very different character from that 
for vision or for smell, for its nerve belongs to the infra-infundibular gToup of 
nerves, and not to the supra-infundibular, as do those of the other two special 
senses. Of the five special senses the nerves for touch, taste, and hearing, all 
belong to the infra-infundibular segmental nerve-groups. The invertebrate 
origin, then, of the vertebrate auditory nerve must be sought for in the infra- 
OBsophageal segmental group of nerves, and not in the supra-cesophageal. 

The organs supplied by the auditory nerve are only partly for the purpose 
of hearing ; there is always present also an apparatus — the semicircular canals 
— concerned with equilibration and co-ordination of movements. Such equili- 
bration organs are not confined to the auditory nerve, but in the water-living 
vertebrates are arranged segmentally along the body, forming the organs of the 
lateral line in fishes ; the auditory organ is but one of these lateral line organs, 
which has been specially developed. 

These lateral line organs have been compared to similar segmental organs 
found in connection with the appendages in worms, especially the respiratory 
appendages. In accordance with this suggestion we see that they are all 
innervated from the region of the respiratory nerves — the vagus, glosso- 
pharyngeal, and facial — nerves which originally supplied the respiratory 
appendages of the palieostracan ancestor. 

The logical conclusion is that the appendages of the Palseostraca possessed 
special sense-organs concerned with the perception of special vibrations, 
especially in the mesosomatic or respiratory region, and that somewhere at the 
junction of the prosoma and mesosoma, one of these sense-organs was specially 
developed to form the origin of the vertebrate auditory apparatus. 

Impressed by this reasoning I made search for some specially striking 
sense-organ at the base of one of the appendages of Limulus, at the junction of 
the prosoma and mesosoma, and was immediately rewarded by the discovery 
of the extraordinary nature of the flabellum, which revealed itself as an 
elaborate sense-organ supplied with a nerve out of all proportion to its size. 
Up to this time no one had the slightest conception that this flabellum was 
a special sense-organ ; the discovery of its nature was entirely due to the 
logical following out of the theory of the origin of vertebrates described in 
this book. 

The structure of this large sense organ is comparable with that of the 
sense-organs of the pectens of the scorpion, and of many other organs found 
on the appendages of various members of the scorpion group, of arachnids and 



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384 THE ORIGIN OF VERTEBRATES 

other air-breathing arthropods. Many of these organs, such as the lyriform 
organs of arachnids, and the * halteres ' or balancers of the Diptera, are usually 
regarded as auditory and equilibration organs. 

On all the mesosomatic appendages of Limulus very remarkable sense -organs 
are found, apparently for estimating pressures, which, when the appendages 
sank into the body to form with their basal parts the branchial diaphragms of 
Ammocoetes, could easily be conceived as remaining at the surface, and so giving 
rise to the lateral line organs. 

Further confirmation of the view that an organ, such as the flabellum, must 
be looked upon as the originator of the vertebrate auditory organ, is afforded by 
the extraordinary coincidence that in Limulus a diverticulum of the generative 
and hepatic mass accompanies the flabellar nerve into the basal part of the digging 
appendage, while in Ammocoetes, accompanying the auditory nerve into the 
auditory capsule, there is seen a mass of cells belonging to that peculiar tissue 
which fills up the space between the brain and the cranial walls, and has already, 
on other grounds, been homologized with the generative and hepatic masses 
which fill up the encephalic region of Limulus. 

For all these reasons special sense-organs, such as are found in the flabellum 
of Limulus and in the pectens of scorpions, may be looked upon as giving 
origin to the vertebrate auditory apparatus. In such case it is highly probable 
that the parachordals, with the auditory capsules attached, arose from a second 
entochondrite of the same nature as the plastron ; a probability which is 
increased by the fact that the scorpion does possess a second entochondrite. 
which, owing to its special relations to the pecten, is known as the supra-pectinal 
entochondrite. 



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CHAPTER XH 

THE REGION OF THE SPINAL CORD 

Difference between cranial and spinal regions. — Absence of lateral root. — 
Meristic variation. — Segmentation of coelom. — Segmental excretory organs. 
— Development of nephric organs ; pronephric, mesonephric, metanepbric. 
— Excretory organs of Ampbioxus. — Solenocytes. — Excretory organs of 
Brancbipus and of Peripatns, appendicular and somatic. — Comparison 
of coBlom of Peripatus and of vertebrate. — Pronepbric organs compared to 
coxal glands.— Origin of vertebrate body-cavity (metacoele). — Segmental 
duct. — Summary of formation of excretory organs.— Origin of somatic 
trunk -musculature. — Atrial cavity of Amphioxus. — Pleural folds. — Ventral 
growth of pleural folds and somatic musculature. — Pleural folds of Cepha- 
laspidse and of Trilobita. — Significance of the ductless glands.— Alteration 
in structure of excretory organs which have lost their duct in vertebrates 
and in invertebrates. — Formation of lymphatic glands. — Segmental coxal 
glands of arthropods and of vertebrates. — Origin of adrenals, pituitary 
body, thymus, tonsils, thyroid, and other ductless glands. — Summary. 

The consideration of the auditory nerve and the auditory apparatus 
terminates the comparison between the cranial nerves of the verte- 
brate and the prosomatic and mesosomatic nerves of the arthropod, 
and leaves us now free to pass on to the consideration of the verte- 
brate spinal nerves and the organs they supply. Before doing so, it 
is advisable to pass in review the conclusions already attained. 

Starting with the working hypothesis that the central nervous # 
system of the vertebrate has arisen from the central nervous system 
of the arthropod, but has involved and enclosed the alimentary canal 
of the latter in the process, so that there has been no reversal of 
surfaces in the derivation of the one form from the other, we have been 
enabled to compare closely all the organs of the head-region in the 
two groups of animals, and in no single case have we been compelled 
to make any startling or improbable assumptions. The simple 
following out of this clue has led in eveiy case in the most natural 

2 o 



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386 THE ORIGIN OF VERTEBRATES 

manner to the interpretation of all the organs in the head-region of 
the vertebrate from the corresponding organs of the arthropod. 

That it is possible to bring together all the striking resemblances 
between organs in the two classes of animals, such as I have done in 
preceding chapters, has been ascribed to a perverted ingenuity on my 
part — a suggestion which is flattering to my imaginative powers, but 
has no foundation of fact. There has been absolutely no ingenuity 
on my part ; all I have done is to compare organs and their nerve- 
supply, as they actually exist in the two groups of animals, on the 
supposition that there has been no turning over on to the back, no 
reversal of dorsal and ventral surfaces. The comparison is there for 
all to read ; it is all so simple, so self-evident that, given the one 
clue, the only ingenuity required is on the part of those who fail 
to see it. 

The great distinction that has arisen between the two head-regions 
is the disappearance of appendages as such, never, however, of 
important organs on those appendages. If the olfactory organs of 
the one group were originally situated on antennules, the olfactory 
organs still remain, although the antennules as such have disap- 
peared. The coxal excretory organs at the base of the endognaths 
remain and become the pituitary body. A special sense-organ, such 
as the flabellum of Limulus or the pecten of scorpion, remains and 
gives rise to the auditory organ. A special glandular organ, the 
uterus in the base of the operculum, remains, and gives rise to the 
thyroid gland. The branchiae and sense-organs on the mesosomatic 
appendages remain, and even the very muscles to a large extent. 
As will be seen later, the excretory organs at the base of the 
metasomatic appendages remain. It is merely the appendage as 
such which vanishes either by dwindling away, or by so great an 
alteration as no longer to be recognizable as an appendage. 

This dwindling process was already in full swing before the 
vertebrate stage ; it is only a continuation of a previous tendency, as 
is seen in the dwindling of the prosomatic appendages in the Mero- 
stomata and the inclusion of the branchiae within the body of the 
scorpion. Already among the Pakeostraca, swimming had largely 
taken the place of crawling. The whole gradual transformation from 
the arthropod to the vertebrate is associated with a transformation 
from a crawling to a swimming animal — with the concomitant loss 
of locomotor appendages as such, and the alteration of the shape of 



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THE REGION OF THE SPINAL CORD 387 

the animal into the lithe fish-like form. The consideration of the 
manner in which this latter change was brought about, takes us 
out of the cranial into the spinal region. 

If we take Limulus as the only living type of the Palieostraca, 
we are struck with the fact that the animal consists to all intents 
and purposes of prosomatic and mesosomatic regions only ; the meta- 
soma consisting of the segments posterior to the mesosoma is very 
insignificant, so that the large mass of the animal consists of what 
has become the head-region in the vertebrate ; the spinal region, 
which has become in the higher vertebrates by far the largest region 
of the body, can hardly be said to exist in such an animal as Limulus. 
As to the Eurypterids and others, similar remarks may be made, 
though not to the same extent, for in them a distinct raetasoma does 
exist. 

In this book I have considered up to the present the cranial 
region as a system of segments, and shown how such segments are 
comparable, one by one, with the corresponding segments in the 
prosoma and mesosoma of the presumed arthropod ancestor. 

In* the spinal region such direct comparison is not possible, as is 
evident on the face of it ; for even among vertebrates themselves the 
spinal segments are not comparable one by one, so great is the varia- 
tion, so unsettled is the number of segments in this region. This 
meristic variation, as Bateson calls it, is the great distinctive character 
of the spinal region, which distinguishes it from the cranial region 
with its fixed number of nerves, and its substantive rather than 
meristic variation. At the borderland, between the two regions, we 
see how the one type merges into the other; how difficult it is 
to fix the segmental position of the spino-occipital nerves ; how much 
more variable in number are the segments supplied by the vagus 
nerves than those anterior to them. 

This meristic variation is a sign of instability, of want of fixedness 
in the type, and is evidence, as already pointed out, that the spinal 
region is newer than the cranial. This instability in the number of 
spinal segments does not necessarily imply a variability in the 
number of segments of the metasoma of the invertebrate ancestor; 
it may simply be an expression of adaptability in the vertebrate 
phylum itself, according to the requirements necessitated by the con- 
version of a crawling into a swimming animal, and the subsequent 
conversion of the swimming into a terrestrial or flying animaL 



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388 THE ORIGIN OF VERTEBRATES 

However many may have been the original number of segments 
belonging to the spinal region, one thing is certain — the segmental 
character of this region is remarkably clearly shown, not only by the 
presence of the segmental spinal nerves, but also by the marked 
segmentation of the mesoblastic structures. The question, therefore, 
that requires elucidation above all others is the origin of the spinal 
mesoblastic segments, i.e. of the coelomic cavities of the trunk-region, 
and the structures derived from their walls. 

Proceeding on the same lines as in the case of the cranial 
segments, it is necessary in the first instance to inquire of the verte- 
brate itself as to the scope of the problem in this region. In addition 
to the variability in the number of segments so characteristic of the 
spinal region, the complete absence in each spinal segment of a 
lateral root affords another marked difference between the two 
regions. Here, except, of course, at the junction of the spinal and 
cranial regions, each segmental nerve arises from two roots only, 
dorsal and ventral, and these roots are separately sensory and motor, 
and not mixed in function as was the lateral root of each cranial 
segment. Now, these lateral roots were originally the nerves sup- 
plying the prosomatic and mesosomatic appendages with motor as 
well as sensory fibres. The absence, therefore, of lateral roots in the 
spinal region implies that in the vertebrate none of the musculature 
belonging to the metasomatic appendages has remained. Conse- 
quently, as far as muscles are concerned, the clue to the origin of 
the spinal segments must be sought for in the segmentation of the 
body-muscles. 

Here, in contradistinction to the cranial region, the segmentation 
is most marked, for the somatic spinal musculature of all vertebrates 
can be traced back to a simple sheet of longitudinal ventral and 
dorsal muscles, such as are seen in all fishes. This sheet is split 
into segments or myotomes by transverse connective tissue septa or 
myo-commata ; each myotome corresponding to one spinal segment. 

In addition to the evidence of segmentation afforded by the body- 
musculature in all the higher vertebrates, similar evidence is given 
by the segmental arrangement of parts of the supporting tissue to 
form vertebrae. Such segments have received the name of sclerotomes, 
and each sclerotome corresponds to one spinal segment. 

Yet another marked peculiarity of this region is the segmental 
arrangement of the excretory organs. Just as our body-musculature 



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THE REGION OF THE SPINAL CORD 389 

has arisen from the uniformly segmented simple longitudinal muscu- 
lature of the lowest fish, so, as we pass down the vertebrate phylum, 
we find more and more of a uniform segmental arrangement in the 
excretory organs. 

The origin of all these three separate segmentations may, in 
accordance with the phraseology of the day, be included in the 
one term — the origin of the spinal mesoblastic segments — i.e. of the 
ccelomic cavities of the trunk-region and the structures derived from 
their walls. 



The Origin of the Segmental Excretory Organs. 

Of these three clues to the past history of the spinal region, the 
segmentation manifested by the presence of vertebrae is the least 
important, for in Ammocoetes there is no sign of vertebrae, and their 
indications only appear at transformation. Especially interesting is 
the segmentation due to the excretory organs, for the evidence dis- 
tinctly shows that such excretory organs have steadily shifted more 
and more posteriorly during the evolution of the vertebrate. 

In Limulus the excretory organs are in the prosomatic region — 
the coxal glands ; these become in the vertebrate the pituitary body. 

In Amphioxus the excretory organs are in the mesosomatic region, 
segmentally arranged with the gills. 

In vertebrates the excretory organs are in the metasomatic region 
posterior to the gills, and are segmentally arranged in this region. 
Their investigation has demonstrated the existence of three distinct 
stages in these organs : 1. A series of segmental excretory organs in 
segments immediately following the branchial segments. This is 
the oldest of the three sets, and to these organs the name of the pro- 
nephros is given. 2. A second series which extends more posteriorly 
than the first, overlaps them to an extent which is not yet settled, 
and takes their place; to them is given the name of the mesone- 
phros. 3. A third series continuous with the mesonephric is situated 
in segments still more posterior, supplants the mesonephros and 
forms the kidneys of all the higher vertebrates. This forms the 
mctancphros. 

These three sets of excretory organs are not exactly alike in their 
origin, in that the prouephric tubules are formed from a different 
portion of the ccelomic walls to that from which the meso- and 



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390 THE ORIGIN OF VERTEBRATES 

metanephric tubules are formed, and the former alone gives origin 
to a duct, which forms the basis for the generative and urinary- 
ducts, and is called the segmental duct. The mesonephric tubules, 
called also the Wolffian body, open into this duct. 

In order to make the embryology of these excretory organs quite 
clear, I will make use of van Wijhe's phraseology and also of his 
illustrations. He terms the whole coelomic cavity the proccelom, 
which is divisible into a ventral unsegmented part, the body-cavity 
or metaccelom, and a dorsal segmented part, the somite. This latter 
part again is divided into a dorsal part — the epimere— and a part 
connecting the dorsal part with the body-cavity, to which therefore 
he gives the name of mesomere. 

The cavity of the epimere disappears, and its walls form the muscle 
and cutis plates of the body. The part which forms the muscles is 
known as the myotome, which separates off from the mesomere, leaving 
the latter as a blind sac — the mesocoelom — communicating by a narrow 
passage with the body cavity or metaccelom. At the same time, from 
the mesomere is formed the sclerotome, which gives rise to the skeletal 
tissues of the vertebra, etc., so that van Wijhe's epimere and mesomere 
together correspond to the original term, pro to vertebra, or somite of 
Balfour; and when the myotome and sclerotome have separated 
off, there is still left the intermediate cell-mass of Balfour and 
Sedgwick, i.e. the sac-like mesoccele of van Wijhe, the walls of which 
give origin to the mesonephrotome or mesonephros. Further, accord- 
ing to van Wijhe, the dorsal part of the unsegmented metaccelom is 
itself segmented, but not, as in the case of the mesoccele, with respect 
to both splanchnopleuric and somatopleuric walls. The segmentation 
is manifest only on the somatopleuric side, and consists of a distinct 
series of hollow somatopleuric outgrowths, called by him hypomercs, 
which give rise to the pronephros and the segmental duct. 

Van Wijhe considers that the whole metaccelom was originally 
segmented, because in the lower vertebrates the segmentation reaches 
further ventral-wards, so that in Selachia the body-cavity is almost 
truly segmental. Also in the gill-region of Amphioxus the cavities 
which are homologous with the body-cavity arise segraentally. 

As is well known, Balfour and Semper were led, from their 
embryological researches, to compare the nephric organs of vertebrates 
with those of annelids, and, indeed, the nature of the vertebrate 
segmental excretory organs has always been the fact which has kept 



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THE REGION OF THE SPINAL CORD 



391 




Fig. 156.— Diagrams to illustrate the Development of the Vertebrate 
Ccelom. (After van Wijhe.) 

N., central nervous system; Nc. t notochord ; Ao., aorta; Mg., midgut. A, My., 
myoccele ; Hies., mesocoele ; Met., metacoele ; Hyp., hypomero (pronephric). B 
and C, My., myotome; Mcs., mesonophros; S.d., segmental duct (pronephric) ; 
Met., body cavity. 



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39 2 THE ORIGIN Of vertebrates 

alive the belief in the origin of vertebrates from a segmented annelid. 
These segmental organs thus compared were the mesonephric tubules, 
and doubts arose, especially in the mind of Gegenbaur, as to the 
validity of such a comparison, because the mesonephric tubules did 
not open to the exterior, but into a duct — the segmental duct — which 
was an unsegmented structure opening into the cloaca ; also because 
the segmental duct, which was the excretory duct of the pronephros, 
was formed first, and the mesonephric tubules only opened into 
it after it was fully formed. Further, the pronephros was said to 
arise from an outbulging of the somatopleuric mesoblast, which 
extended over a.limited number of metameres, and was not segmental, 
but continuous. Gegenbaur and others therefore argued that the 
original prevertebrate excretory organ was the pronephros and its duct, 
not the mesonephros, from which they concluded that the vertebrate 
must have been derived from an unsegmented type of animal, and 
not from the segmented annelid type. 

Such a view, however, has no further reason for acceptance, as 
it was based on wrong premises, for Eiickert has shown that the 
pronephros does arise as a series of segmental nephric tubules, and 
is not unsegmented. He also has pointed out that in Torpedo the 
anterior part of the pronephric duct shows indications of being seg- 
mented, a statement fully borne out by the researches of Maas on 
Myxine, who gives the clearest evidence that in this animal the 
anterior part of the pronephric duct is formed by the fusion of a 
series of separate ducts, each of which in all probability once 
opened out separately to the exterior. 

Eiickert therefore concludes that Balfour and Semper were right 
in deriving the segmental organs of vertebrates from those of annelids, 
but that the annelid organs are represented in the vertebrate, not by 
the mesonephric tubules, but by the pronephric tubules and their 
ducts, which originally opened separately to the exterior. By the 
fusion of such tubules the anterior part of the segmental duct was 
formed, while its posterior part either arose by a later ccenogenetic 
lengthening, or is the only remnant of a series of pronephric tubules 
which originally extended the whole length of the body, as suggested 
also by Maas and Boveri. Eiickert therefore supposed that the 
mesonephric tubules were a secondary set of nephric organs, which 
were not necessarily directly derived from the annelid nephric 
organs. 



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THE REGION OF THE SPINAL CORD 393 

At present, then, Ruckert's view is the one most generally ac- 
cepted — the original annelid nephric organs are represented by the 
pronephric tubules and the pronephric duct, not by the mesonephric 
tubules, which are a later formation. This latter statement would 
hold good if the mesonephric tubules were found entirely in seg- 
ments posterior to those containing the pronephric tubules; such, 
however, is said not to be the case, for the two sets of orgaus are 
said to overlap in some cases; even when they exist in the same 
segments, the former are said always to be formed from a more 
dorsal part of the coelom than the pronephros, always to be a later 
formation, and never to give any indication of communicating with 
the exterior except by way of the pronephric duct. 

The recent observations of Brauer on the excretory organs of the 
Gymnophiona throw great doubt on the existence of mesonephric and 
pronephric tubules in the same segment. He criticizes the observa- 
tions on which such statements are based, and concludes that, as in 
Hypogeophis, the nephrotome which is cut off after the separation of 
the sclero-myotome gives origin to the pronephros in the more anterior 
regions, just as it gives origin to the mesonephros in the more 
posterior regions. In fact, the observations of van Wijhe and others 
do not in reality show that two excretory organs may be formed 
in one segment, the one mesonephric from the remains of the nieso- 
mere and the other pronephric from the hypomere, but rather that 
in such cases there is only one organ — the pronephros — part of which 
is formed from the mesomere and part from the hypomere. Brauer 
goes further than this, and doubts the validity of any distinction 
between pronephros and mesonephros, on the ground of the former 
arising from a more ventral part of the procculom than the latter ; 
for, as he says, it is only possible to speak of one part of the somite 
as being more ventral than another part when both parts are in the 
same segment; so that if pronephric and mesonephric organs are 
never in the same segment, we cannot say with certainty that the 
former arises more ventrally than the latter. 

These observations of Brauer strongly confirm Sedgwick's original 
statement that the pronephric and mesonephric organs are hoino- 
dynamous organs, in that they are both derived from the original 
serially situated nephric organs, the differences between them being 
of a subordinate nature and not sufficient to force us to believe that 
the mesonephros is an organ of quite different origin to the 



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394 THE ORIGIN OF VERTEBRATES 

pronephros. So, also, Price, from his investigations of the excretory 
organs of Bdellostoma, considers that in this animal both pro- 
nephros and mesonephros are derived from a common embryonic 
kidney, to which he gives the name holoncphros. 

Brauer also is among those who conclude that the vertebrate 
excretory organs were derived from those of annelids ; he thinks that 
the original ancestor possessed a series of similar organs over the 
whole pronephric and mesonephric regions, and that the anterior 
pronephric organs, which alone form the segmental duct, became 
modified for a larval existence — that their peculiarities were adaptive 
rather than ancestral. This last view seems to me very far-fetched, 
without any sufficient basis for its acceptance. According to the 
much more probable and reasonable view, the pronephros represents 
the oldest and original excretory organs, while the mesonephros 
is a later formation. Brauer's evidence seems to me to signify that 
the pronephros, mesonephros, and metanephros are all serially homo- 
logous, and that the pronephros bears much the same relation to 
the mesonephros that the mesonephros does to the metanephros. 
The great distinction of the pronephros is that it, and it alone, 
forms the segmental duct 

We may sum up the conclusions at which we have now arrived 
as follows : — 

1. The pronephric tubules and the pronephric duct are the oldest 
part of the excretory system, and are distinctly in evidence for a 
few segments only in the most anterior part of the trunk-region 
immediately following the branchial region. They differ also from 
the mesonephric tubules by not being so clearly segmental with the 
myotomes. 

2. The mesonephric tubules belong to segments posterior to those 
of the pronephros, are strictly segmental with the myotomes, and 
open into the pronephric duct. 

3. All observers are agreed that the two sets of excretory organs 
resemble each other in very many respects, as though they arose 
from the same series of primitive organs, and, according to Sedgwick 
and Brauer, no distinction of any importance does exist between 
the two sets of organs. Other observers, however, consider that the 
pronephric organs, in part at all events, arise from a part of the 
nephrocele more ventral than that which gives origin to the mesone- 
phric organs, and that this difference in position of origin, combined 



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THE REGION OF THE SPINAL CORD 395 

with the formation of the segmental duct, does constitute a true 
morphological distinction between the two sets of organs. 

4. All the recent observers are in agreement that the vertebrate 
excretory organs strongly indicate a derivation from the segmental 
organs of annelids. 

The very strongest support has been given to this last conclusion 
by the recent discoveries of Boveri and Goodrich upon the excretory 
organs of Amphioxus. According to Boveri, the nephric tubules of 
Amphioxus open into the dorsal ccelom by one or more funnels. 
Around each funnel are situated groups of peculiar cells, called by 
him ' Fadenzellen/ each of which sends a long process across the 
opening of the funnel. Goodrich has examined these ' Fadenzellen/ 
and found that they are typical pipe-cells, or solenocytes, such as he 
has described in the nephridial organs of various members of the 
annelid group Polychaeta. Also, just as in the Polyclueta, the ciliated 
nephric tubule has no internal funnel-shaped opening into the ccelom, 
but terminates in these groups of solenocytes. "Each solenocyte 
consists of a cell-body and nucleus situated at the distal free 
extremity of a delicate tube ; the proximal end of the tube pierces 
the wall of the nephridial canal and opens into its lumen. A siDgle 
long flagellum arising from the cells works in the tube and projects 
into the canal." 

The exceedingly close resemblance between the organs of 
Amphioxus and those of Phyllodoce, as given in his paper, is most 
striking, and, as he says, leads to the conclusion that the excretory 
organs of Amphioxus are essentially identical with the nephridia of 
certain polychaete worms. 

It is to me most interesting to find that the very group of 
annelids, the Polyclueta, which possess solenocytes so remarkably 
resembling those of the excretory organs of Amphioxus, are the 
highest and most developed of all the Annelida. I have argued 
throughout that the law of evolution consists in the origination of 
successive forms from the dominant group then alive, dominance 
signifying the highest type of brain-power achieved up to that time. 
The highest type among Annelida is found in the Chrctopoda; from 
them, therefore, the original arthropod type must have sprung. 
This original group of Arthropoda gave rise to the two groups of 
Crustacea and Arachnida, in my opinion also to the Vertebrata, 
and, as already mentioned, it is convenient to give it a generalized 



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396 THE ORIGIN OF VERTEBRATES 

name, the Protostraca, from' which subsequently the Paheostraca 
arose. 

The similarity between the excretory organs of Amphioxus and 
those of Phyllodoce suggests that the protostracan ancestor of the 
vertebrates arose from the highest group of the Chsetopoda — the 
Polychseta. The evidence which I have already given points, how- 
ever, strongly to the conclusion that the vertebrate did not arise from 
members of the Protostraca near to the polychaete stock, but rather 
from members in which the arthropod characters had already become 
well developed— members, therefore, which were nearer the Trilobita 
than the Polychseta. Such early arthropods would very probably 
have retained in part excretory organs of the same character as those 
found in the original polychaete stock, and thus account for the 
presence of solenocytes in the excretory organs of Amphioxus. 

In connection with such a possibility, I should like to draw 
jattention to the observations of Claus and Spangenberg on the 
lexcretory organs of Branchipus — that primitive phyllopod, which is 
[recognized as the nearest approach to the trilobites at present living. 
According to Claus, an excretory apparatus exists in the neighbour- 
hood of each nerve-ganglion, and Spangenberg finds a perfectly 
similar organ in the basal segment of each appendage — a system, 
therefore, of excretory organs as segmentally arranged as those of 
Peripatus. Claus considers that although these organs formed an 
excretory system, it is not possible to compare them with the 
annelid segmental organs, because he thought the cells in question 
arose from ectoderm. Now, the striking point in the description of 
the excretory cells in these organs, as described both by Claus and 
Spangenberg, is that they closely resemble the pipe-cells or sole- 
nocytes of Goodrich ; each cell possesses a long tube-like projection, 
which opens on the surface. They appear distinctly to belong to the 
category of flame-cells, and resemble solenocytes more than anything 
else. According to Goodrich, the solenocy te is probably an ectodermal 
cell, so that even if it prove to be the case, as Claus thought, that 
these pipe-cells of Branchipus are ectodermal, they would still claim 
to be derived from the segmental organs of annelids, especially of the 
Polycheeta, being, to use Goodrich's nomenclature, true nephridial 
organs, as opposed to ccelomostomes. 

These observations of Claus and Spangenberg suggest not only 
that the primitive arthropod of the trilobite type possessed segmental 



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THE REGION OF THE SPINAL CORD 397 

organs in every segment directly derived from those of a polychsete 
ancestor, but also that such organs were partly somatic and partly 
appendicular in position. Such a suggestion is in strict accord with 
the observations of Sedgwick on the excretory organs of the most primi- 
tive arthropod known, viz. Peripatus, where also the excretory organs, 
which are true segmental organs, are partly somatic and partly 
appendicular. Further, the excretory organs of the Scorpion and 
Limulus group are again partly somatic and partly appendicular, 
receiving the name of coxal glands, because there is a ventral projec- 
tion of the gland into the coxa of the corresponding appendage. 

Judging from all the evidence available, it is probable that when 
the arthropod stock arose from the annelids, simultaneously with the 
formation of appendages, the segmental somatic nephric organs of 
the latter extended ventrally into the appendage, and thus formed 
a segmental set of excretory organs, which were partly somatic, 
partly appendicular in position, and might therefore be called coxal 
glands. 

As already stated, all investigators of the origin of the vertebrate 
excretory organs are unanimous in considering them to be derived 
from segmental organs of the annelid type. I naturally agree with 
them, but, in accordance with my theory, would substitute the words 
" primitive arthropod " for the word " annelid," for all the evi- 
dence I have accumulated in the preceding chapters points directly 
to that conclusion. Further, the most primitive of the three sets 
of vertebrate segmental organs — the pronephros, mesonephros, and 
metanephros — is undoubtedly the pronephros ; consequently the 
pronephric tubules are those which I consider to be more directly 
derived from the coxal glands of the primitive arthropod ancestor. 
Such a derivation appears to me to afford an explanation of the diffi- 
culties connected with the origin of the pronephros and mesonephros 
respectively, which is more satisfactory than that given by the direct 
derivation from the annelid. 

The only living animal which we know of as at all approaching 
the most primitive arthropod type is, as pointed out by Korschelt and 
Heider, Peripatus ; and Peripatus, as is well known, possesses a true 
coelom and true ccelomic excretory organs in all the segments of the 
body. Sedgwick shows that at first a true coelom, as typical as that 
of the annelids, is formed in each segment of the body, and that then 
this cceloni (which represents in the vertebrate van Wijhe's pro-ccelom) 



1 



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398 THE ORIGIN OF VERTEBRATES 

splits into a dorsal and a ventral part. In the anterior segments of 
the body the dorsal part disappears (presumably its walls give origin 
to the mesoblast from which the dorsal body-muscles arise), while 
the ventral part remains and forms a nephroccele, giving origin to the 
excretory organs of the adult. According to von Kennel, the cavity 
becomes divided into three spaces, which for a time are in com- 
munication — a lateral (I.), a median (II.), and a dorso-median (III.). 
The dorso-median portion becomes partitioned off, and this, as well 
as the greater part of the lateral portion, which lies principally in 
the foot, is used up in providing elements for the formation of the 
body- and appendage-muscles respectively and the connective tissue. 

In Fig. 157 I reproduce von Kennel's diagram of a section across 
a Peripatus embryo, in which I. represents the lateral appendicular 
part of the cceloni, II. the ventral somatic part, and III. the dorsal 
part which separates off from the ventral and lateral parts, and, as 
its walls give origin largely to the body-muscles, may be called 
the myocoele. The muscles of the appendages are formed from 
the ventral part of the original proccelom, just as I have argued 
is the case with the muscles of the splanchnic segmentation in 
vertebrates. 

Sedgwick states that the ventral part of the ccelom extends 
into the base of each appendage, and there forms the end-sac of 
each nephric tubule, into which the nephric funnel opens, thus 
forming a coxal gland; this end-sac or vesicle in the appendage 
is called by him the internal vesicle (i.v.), because later another 
vesicle is formed from the ventral ccelom in the body itself, close 
against the nerve-cord on each side, which he calls the external 
vesicle (e.v.). (Cf. Fig. 158, taken from Sedgwick.) This second 
vesicle is, according to him, formed later in the development from 
the nephric tubule of the internal vesicle, so that it discharges 
its contents to the exterior by the same opening as the original 
tubule. Of course, as he points out, the whole system of internal 
and external vesicles and nephric tubules are all simply derivatives 
of the original ventral part of the ccelom or nephroccele. 

Here, then, in Peripatus, and presumably, therefore, in members 
of the Protostraca, we see that the original segmental organs of the 
annelid have become a series of nephric organs, which extended into 
the base of the appendages, and may therefore be called coxal glands ; 
also it is clear, from Sedgwick's description, that if the appendages 



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THE REGION OF THE SPINAL CORD 



399 



disappeared, the nephric organs would still remain, not as coxal 
glands, but as purely somatic excretory glands. They would still be 
homologous with the annelid segmental organs, or with the coxal 
glands, but would arise in toto from a part of the ventral ccelom or 
nephrocele, more dorsal than the former appendicular part, because 
the appendages and their enclosed ccelom are always situated ventrally 
to the body. Again, according to Sedgwick, the nephric tubules are 




App 



Fig. 157.— Transverse Section of Peripatus Embryo. (After von Kennel.) 

AL, alimentary canal; N., nerve-cord; App., appendage; I, II, III, the three 
divisions (lateral, median, and dorso-median) of the ccelom. 




t V 

App 



Fig. 158. — Section of Peripatus. (After Sedgwick.) 

Ah, alimentary canal; N., nerve-cord; App., appendage; i.v., internal, and e.v., 
external vesicles of the segmented excretory tubule (coxal gland). 

connected with two cfplomic vesicles, the one in the appendage the 
internal vesicle, and the other, the so-called bladder, or the external 
vesicle, in the body itself, close against the nerve-cord. Sedgwick 
appears to consider that either of these vesicles may form the end- 
sac of a nephric tubule, for he discusses the question whether the 
single vesicle, which in each case gives origin to the nephridia of the 
first three legs, corresponds to the internal or external vesicle. He 



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400 THE ORIGIN OF VERTEBRATES 

decides, it is true, in favour of the internal vesicle, and therefore 
considers the excretory organ to be appendicular, i.e. a coxal gland, in 
these segments as well as in those more posterior. Still, the very 
discussion shows that in his opinion, at all events, the external 
vesicle might represent the end-sac of the tubule, in the absence of 
the internal or appendicular vesicle. 

Such an arrangement as Sedgwick describes in Peripatus is the 
very condition required to give rise to the pronephric and meso- 
nephric tubules, as deduced by me from the consideration of the 
vertebrate, and harmonizes and clears up the controversy about the 
mesonephros and pronephros in the most satisfactory manner. Both 
I pronephros and mesonephros are seen to be derivatives of the original 
annelid segmental oigans, not directly from an annelid, but by way 
j of an arthropodan ancestor; the difference between the two is 
simply that the pronephric organs were coxal glands, and indi- 
cate, therefore, the presence of the original metasomatic appendages, 
while the mesonephric organs were homologous organs, formed in 
segments of later origin which had lost their appendages. For this 
reason the pronephros is said to be formed, in part at least, from 
a portion of the coelom situated more ventrally than the purely 
somatic part which gives rise to the mesonephros. For this reason 
Sedgwick, Brauer, etc., can say that the mesonephros is strictly homo- 
dynamous with the pronephros; while equally Kiickert, Semon, and van. 
Wijhe can say it is not homodynamous, in so far that the two organs 
are not derived strictly from absolutely homologous parts of the coelom. 
For this reason Semon can speak of the mesonephros as a dorsal 
derivative of the pronephros, just as Sedgwick says that the external 
or somatic vesicle of Peripatus is a derivative of the appendicular 
nephric organ. For this reason the pronephros, or rather a part of it, 
is always derived from the somatopleuric layer, for, as is clear from 
Miss Sheldon's drawing, the part of the cceloni in Peripatus wliich 
dips into the appendage is derived from the somatopleuric layer 
alone. 

Such a ccelom as that of Peripatus, Fig. 157, would represent the 
origin of the vertebrate ccelom, and would therefore represent the 
procrolom of van Wijhe. In strict accordance with this, we see that it 
separates into a dorsal part, the walls of which give origin to the 
somatic muscles, or at all events to the great longitudinal dorsal 
muscles of the animal, and a ventral part, which forms a nephrocele, 



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THE REGION OF THE SPINAL CORD 401 

dips into the appendage, and gives origin to the muscles of the 
appendage. In the vertebrate, after the somatic dorsal part or 
myoccele has separated off, a ventral part is left, which forms a 
nephrocoele in the trunk-region, and gives origin to the splanchnic 
striated muscles in the cranial region, i.e. to the muscles which, 
according to my theory, were once appendicular muscles. This 
ventral nephroccelic part is divisible in the trunk into a segmented 
part, which forms the excretory organs proper, and an unsegmented 
part, the metaccele or true body-cavity of the vertebrate. 

This comparison of the proccelom of the vertebrate and arthropod 
signifies that the vertebrate metacoele was directly derived by ventral 
downgrowth from the arthropod nephrocoele, so that if, as I suppose, 
the vertebrate nervous system represents the conjoined nervous 
system and alimentary canal of the arthropod, then the vertebrate 
metaccele, or body-cavity, must have been originally confined to the 
region on each side of the central nervous system, and from this 
position have spread ventrally, to enclose ultimately the new-formed 
vertebrate gut. This means that the body-cavity (metacoele) of the 
vertebrate is not the same as the body-cavity of the annelid, but 
corresponds to a ventral extension of the nephrocoele, or ventral part 
of such body-cavity. 

Such a phylogenetic history is most probable, because it explains 
most naturally and simply the facts of the development of the verte- 
brate body-cavity ; for the mesoblast always originates in the neigh- 
bourhood of the notochord and central nervous system, and the lumen 
of the body- cavity always appears first in that region, and then 
extends laterally and ventrally on each side until it reaches the most 
ventral surface of the embryo, thus forming a ventral mesentery, 
which ultimately disappears, and the body-cavity surrounds the gut, 
except for the dorsal mesentery. Thus Shipley, in his description of 
the formation of the mesoblast ic plates which line the body- cavity in 
Ammocoetes, describes them as commencing in two bands of meso- 
blast situated on each side, close against the commencing nervous 
system: — 

" These two bands are separated dorsally by the juxtaposition of 
the dorsal wall of the mesenteron and the epiblast, and ventrally by 
the hypoblastic yolk- cells which are in contact with the epiblast 
over two-thirds of the embryo. Subsequently, but at a much later 
date, the mesoblast is completed ventrally by the downgrowth on 

2 D 



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402 THE ORIGIN OF VERTEBRATES 

each side of these mesoblastic plates. The subsequent downward 
growth is brought about by the cells proliferating along the free 
ventral edge of the mesoblast, these cells then growing ventral wards, 
pushing their way between the yoke-cells and epiblast." 

The derivation of the vertebrate pronephric segmental organs 
from the metasomatic coxal glands of a primitive arthropod would 
mean, if the segmental organs of Peripatus be taken as the type, 
that such glands opened to the exterior on every segment, either 
at the base of the appendage or on the appendage itself. It is 
taken for granted by most observers that the pronephric segmental 
organs once opened to the exterior on each segment, and then, 
from some cause or other, ceased to do so, and the separate ducts, 
by a process of fusion, came to form a single segmental duct, which 
opened into the cloaca. Many observers have been led to the con- 
clusion that the pronephric duct is epiblastic in origin, although 
from its position in the adult, it appears far removed from all 
epiblastic formations. However, at no time in the developmental 
history is there any clear evidence of actual fusion of any part of the 
pronephric organ with the epidermis, and the latest observer, Brauer, 
is strongly of opinion that there is never sufficiently close contact 
with the epidermis to warrant the statement that the epiblastic cells 
take part in the formation of the* duct. All that can be said is, that 
the formation of the duct takes place at a time when the pronephric 
diverticulum is in close propinquity to the epidermis, before the 
ventral downgrowth of the myotome has taken place. 

The formation of the anterior portion of the pronephric duct is, 
according to Maas in Myxine, and Wheeler in Petromyzon, undoubtedly 
brought about by the fusion of a number of pronephric tubules, which, 
according to Maas, are clearly seen in the youngest specimens as 
separate segmental tubes; each of these tubules is supplied by a 
capillary network from a segmental branch of the aorta, as in the 
tubules of Amphioxus according to Boveri, and does not possess a 
glomerulus. 

The posterior part of the duct into which the mesonephric tubules 
enter possesses also a capillary network, which Maas considers to 
represent the original capillary network of a series of pronephric 
tubules, the only remnant of which is the duct into which the 
mesonephric tubules open. He therefore argues that the pronephric 
duct indicates a series of pronephric tubules, which originally extended 



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THE REGION OF THE SPINAL CORD 403 

along the whole length of the body, and were supplanted by the 
mesonephric tubules, which also belonged to the same segments. 

I also think that the paired appendages which have left the pro- 
nephric tubules as signs of their past existence, existed originally, in 
the invertebrate stage, on every segment of the body. But I do not 
consider that such a statement is at all equivalent to saying that such 
pairs of tubules must have existed upon every one of the segments' 
existing at the present day ; for it seems to me that Ruckert is much 
more likely to be right when he says that in Selachians the duct 
clearly does grow back, and is not formed throughout in situ.; so that 
he gives a double explanation of the formation of the duct — a palin- 
genetic anterior part formed by the fusion of the extremities of the 
original excretory tubules, to which a posterior coenogenetic lengthen- 
ing has been added. 

It does not seem to me at all necessary that the immediate inver- 
tebrate ancestor of the vertebrate should have possessed excretory 
organs which opened out separately to the exterior on each segment ; 
a fusion may already have taken place in the invertebrate stage, and 
so a single duct have been acquired for a number of organs. Such a 
suggestion has been made by Eiickert, because of the fact discovered 
by Cunningham and E. Meyer, that the segmental organs of Lanice 
conchilega are on each side connected together by a single strong 
longitudinal canal. I would, however, go further than this and say, 
that even although the nephric organs of the polychaete ancestor 
opened out on every segment, and although the primitive arthropodan 
ancestor derived from such polychaete possessed coxal glands which 
opened out either on to or at the base of each appendage, similarly to 
those of Peripatus, yet the immediate arthropodan ancestor, with its 
paleeostracan affinities, may already have possessed metasomatic coxal 
glands, all of which opened into a single duct, with a single opening 
to the exterior. 

Judging from Limulus, such was very probably the case, for 
Patten and Hazen have shown (1) that the coxal glands of Limulus 
are segmental organs belonging to the prosomatic segments ; (2)* that 
the organs belonging to the cheliceral and ectognathal segments 
are not developed ; (3) that the four glands belonging to the endo- 
gnaths become connected together by a stolon, which communicates 
with a single nephric duct, opening to the exterior on the basal 
segment of the 5th prosomatic appendage (the last endognath). At 



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404 THE ORIGIN OF VERTEBRATES 

no time is there any evidence of any separate openings or any fusion 
with the ectoderm, such as might indicate separate openings of these 
prosomatic coxal segmental organs. Thus we see that in Limulus, 
which is presumably much nearer the annelid condition than the 
vertebrate, all evidence of separate nephric ducts opening to the 
exterior on each prosomatic segment has entirely disappeared, just as 
is the case in the metasomatic coxal glands (i.c. the pronephros) of 
the vertebrate. What is seen in the prosomatic region of Limulus, 
and doubtless also of the Eurypterids, may very probably have 
occurred in the metasomatic region of the immediate invertebrate 
ancestors of the vertebrate, and so account for the single pro- 
nephric duct belonging to a number of pronephric organs. 

The interpretation of these various embryological investigations 
may be summed up as follows : — 

I 1. The ancestor of the vertebrates possessed a pair of appendages 
on each segment ; into the base of each of these appendages the 
segmental excretory organ sent a diverticulum, thus forming a coxal 
gland. 

2. Such coxal glands, even in the invertebrate stage, may have 
discharged into a common duct which opened to the exterior most 
posteriorly. 

3. Then, from some cause, the appendages were rendered useless, 
and dwindled away, leaving only the pronephric orgaus to indicate 
their former presence. At the end of this stage the animal possessed 
vertebrate characteristics. 

4. For the purpose of increasing mobility, of forming an efficient 
swimming instead of a crawling animal, the body-segments increased 
in number, always, as is invariably the case, by the formation of new 
ones between those already formed and the cloacal region, and so of 
necessity caused an elongation of the pronephric duct. Into this there 
now opened the ducts of the segmental organs formed by recapitula- 
tion, those, therefore, belonging to the body-segments — mesonephric — 
having nothing to do with appendages, for the latter had already 
cease'd to exist functionally, and would not, therefore, be repeated with 
each meristic repetition. 

This, so to speak, passive lengthening of the pronephric duct in 
consequence of the lengthening of the early vertebrate body by the 
addition of metameres, each of which contained only mesonephric 
and no pronephric tubules, is, to my mind, an example of a principle 



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THE REGION OF THE SPINAL CORD 405 

which has played an important part in the formation of the verte- 
brate, viz. that the meristic variation by which the spinal region of 
even the lowest of existing vertebrates has been formed, has largely 
taken place in the vertebrate phylum itself, and that such changes 
must be eliminated before we can picture to ourselves the pre-verte- 
brate condition. As an example, I may mention the remarkable 
repetition of similar segments pictured by Bashford Dean in Bdello- 
stoma. Such repetition leads to passive lengthening of such parts 
as are already formed but are not meristically repeated : such are the 
notochord, the vertebrate intestine, the canal of the spinal cord, and 
possibly the lateral line nerve. The fuller discussion of this point 
means the discussion of the formation of the vertebrate alimentary 
canal; I will therefore leave it until I come to that part of my 
subject, and only say here that the evidence seems to me to point to 
the conclusion that at the time when the vertebrate was formed, the 
respiratory and cloacal regions were very close together, the whole of 
the metasoma being represented by the region of the pronephros 
alone. 

Here, as always, the evidence of Ammoccetes tends to give 
definiteness to our conceptions, for Wheeler points out that up to a 
length of 7 mm. the pronephros only is formed ; there is no sign of 
the more posteriorly formed mesonephros. Now we know, as pointed 
out in Chapter VI., p. 228, this is the time of Kupffer's larval stage 
of Ammoccetes. This is the period during which the invertebrate 
stage is indicated in the ontogeny, so that, in accordance with all 
that has gone before, this means that the metasoma of the inverte- 
brate ancestor was confined to the region of the pronephros. 

Again, take Shipley's account of the development of Petromyzon. 
He says — 

"The alimentary canal behind the branchial region may be 
divided into three sections. Langerhans has termed these the stomach, 
midgut, and hindgut, but as the most anterior of these is the narrowest 
part of the whole intestine, it would, perhaps, be better to call it 
oesophagus. This part of the alimentary canal lies entirely in front 
of the yolk, and is, with the anterior region, which subsequently 
bears the gills, raised from the rest of the egg when the head is 
folded off. It is supported by a dorsal mesentery, on each side of 
which lies the head-kidney (pronephros). " 

Further on he says — 



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406 THE ORIGIN OF VERTEBRATES 

"The hindgut is smaller than the midgut; its anteri6r limit is 
marked by the termination of the spiral valve, which does not extend 
into this region. The two segmental ducts open into it just where it 
turns ventrally to open to the exterior by a median ventral anus. 
Its lumen is from an early stage lined with cells which have lost 
their yolk, and it is in wide communication with the exterior from 
the first. This condition seems to be, as Scott suggests, connected 
with the openings of the ducts of the pronephros, for this gland is 
completed and seems capable of functioning long before any food 
could find its way through the midgut, or, indeed, before the stomo- 
dieum has opened." 

Is there no significance in this statement of Shipley ? Even if it 
be possible to find some special reason why the branchial and cloacal 
parts of the gut are freed from yolk and lined with serviceable 
epithelium a long time before the midgut, why should a bit of the 
midgut, which Shipley calls the oesophagus, which is connected with 
the region of the pronephros and not of the branchiae, differ so 
markedly from the rest of the midgut ? Surely the reason is that 
the branchial region of the gut, the pronephric region of the gut, and 
the cloacal region of the gut, belong to a different and earlier phase 
in the phylogenetic history of the Ammocoetes than does the midgut 
between the pronephric and cloacal rejions. This observation of 
Shipley fits in with and emphasizes the view that the original animal 
from which the vertebrate arose consisted of a cephalic and branchial 
region, followed by a pronephric and cloacal region ; the whole inter- 
mediate part of the gut, which forms the midgut, with its large lumen 
and spiral valve, and belongs to the mesonephric region, being a later 
formation brought about by the necessity of increasing the length of 
the body. 

The Origin of the Somatic Trunk-Musculature and the 
Formation of an Atrial Cavity. 

Next comes the question, why was the pronephros not repeated 
in the meristic repetition that took place during the early vertebrate 
stage ? What, in fact, caused the disappearance of the metasomatic 
appendages, and the formation of the smooth body-surface of the fish ? 

The embryological evidence given by van Wijhe and others of 
the manner in which the original superficially situated pronephros is 



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THE REGION OF THE SPINAL CORD 407 

removed from the surface and caused to assume the deeper position, 
as seen ia the later embryo, is perfectly clear and uniform in all the 
vertebrate groups. The diagrams at the end of van Wijhe's paper, 
which I reproduce here, illustrate the process which takes place. At 
first the myotome (Fig. 159, A) is confined to the dorsal region on 
each side of the spinal cord and notochord. Then (Fig. 159, B) it 
separates from the rest of the somite and commences to extend ven- 
trally, thus covering over the pronephros and its duct, until finally 
(Fig. 159, C) it reaches the mid- ventral line on each side,. and the 
foundations of the great somatic body-muscles are finally laid. 

In order, therefore, to understand how the obliteration of the 
appendages took place, we must first find out what is the past history 
of the myotomes. Why are they confined at first to the dorsal region 
of the body, and extend afterwards to the ventral region, forcing by 
their growth an organ that was originally external in situation to 
become internal ? 

In the original discussion at Cambridge, I was accused of violating 
the important principle that in phylogeny we must look at the most 
elementary of the animals whose ancestors we seek, and was told 
that the lowest vertebrate was Amphioxus, not Ammocoetes; that 
therefore any argument as to the origin of vertebrates must proceed 
from the consideration of the former and not the latter animal. My 
reply was then, and is still, that I was considering the cranial region 
in the first place, and that therefore it was necessary to take the 
lowest vertebrate which possessed cranial nerves and sense-organs of 
a distinctly vertebrate character, a criterion evidently not possessed 
by Amphioxus. Such argument does not apply to the spinal region, 
so that, now that I have left the cranial region and am considering 
the spinal, I entirely agree with my critics that Amphioxus is likely 
to afford valuable help, and ought to be taken into consideration as 
well as Ammocoetes. The distinction between the value of the spinal 
(including respiratory) and cranial regions of Amphioxus for drawing 
phylogenetic conclusions is recognized by Boveri, who says that, in 
his opinion, " Amphioxus shows simplicity and undifferentiation 
rather than degeneration. If truly Amphioxus is somewhat degene- 
rated, then it is so in its prehensile and masticatory apparatus, its 
sense organs, and perhaps its locomotor organs, owing to its method 
of living." 

Hatschek describes in Amphioxus how the ccelom splits into a 



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4o8 



THE ORIGIN OF VERTEBRATES 




Fi(i. 15!).— Diagrams to illustrate the Development of the Vertebrate 
Ccelom. (After van Wijhe.) 

A'., con t nil nervous nystem ; AY., notochord ; Ao. t aorta ; Mg., midgut. A, My., 
myocofle; Mes., mesoccele; Met., metacoele; Hyp., hypomcro (pronephric). B 
and C, My., myotome ; Mes., mesonephros ; S.d., tegmental duct (pronephric) ; 
Met., body-cavity. 



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THE REGION OF THE SPINAL CORD 409 

dorsal segmented portion, the protovertebra, and a ventral unseg- 
mented portion, the lateral plates. He describes in the dorsal part 
the formation of myotome and sclerotome, as in the Craniota. 
Also, he describes how the myotome is at first confined to the dorsal 
region in the neighbourhood of the spinal cord and notochord, and 
subsequently extends ventrally, until, just as in Ammoccetes, the 
body is enveloped in a sheet of somatic segmented muscles, the well- 
known myomeres. 

The conclusion to be drawn from this is inevitable. Any explana- 
tion of the origin of the somatic muscles in Ammoccetes must also 
be an explanation of the somatic muscles in Amphioxus, and con- 
versely ; so that if in this respect Amphioxus is the more primitive 
and simpler, then the condition in Ammocoetes must be looked upon 
as derived from a more primitive condition, similar to that found in 
Amphioxus. Now, it is well know that a most important distinction 
exists between Amphioxus and Ammoccetes in the topographical 
relation of the ventral portion of this muscle-sheet, for in the former 
it is separated from the gut and the body-cavity by the atrial space, 
while in the latter there is no such space. Fiirbringer therefore 
concludes, as I have already mentioned, that this space has become 
obliterated in the Craniota, but that it must be taken into considera- 
tion in any attempt at formulating the nature of the ancestors of the 
vertebrate. 

Kowalewsky described this atrial space as formed by the ventral 
downgrowth of pleural folds on each side of the body, which met in 
the mid-ventral line and enclosed the branchial portion of the gut. 
According to this explanation, the whole ventral portion of the 
somatic musculature of the adult Amphioxus belongs to the extension 
of the pleural folds, the original body-musculature being confined to 
the dorsal region. This is expressed roughly on the external surface 
of Amphioxus by the direction of the connective tissue septa between 
the myotomes (cf. Fig. 162, B). These septa, as is well known, bend 
at an angle, the apex of which points towards the head. The part 
dorsal to the bend represents the part of the muscle belonging to the 
original body ; the part ventral to the bend is the pleural part, and 
represents the extension into the pleural folds. 

Lankester and Willey have attempted to give another explanation 
of the formation of the atrial cavity ; they look upon it as originating 
from a ventral groove, which becomes a canal by the meeting of two 



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